One aspect to this question is that of the age at which one responds to a call to a particular vocation. Perhaps for good reason, there has been a trend to accepting people into religious formation at an older age than in the past. Entering formation at an early age does clearly have the risk that such entry is seen as definitive, and those who find that they should leave and test a different vocation find it difficult to do so. But, on the other hand, if entry is left until later an opposite risk occurs. An older person can be more set in their ways, and therefore less docile and less adaptable to the formative demands of a particular form of life. I found this post at Southwark Vocations interesting, and would be supportive of its implication that we should perhaps be open to encouraging a response to vocation at an earlier age.
This question of the age at which one responds to a vocation is linked to that of how we understand the single state in the life of the Church. I happen to think that the single state, precisely as a single state (but see comments below about lay movements and ecclesial communities), is not a vocational choice in itself, and should not be presented as such in pastoral activity. It represents an openess towards a choice of vocation in the Church. So the situation of someone who remains single until late in life has something unfinished about it, something anomalous. This what Adrienne von Speyr writes about this:
The call reaches persons in their youth, and also older persons. But the "late called" are mostly those with whom the Lord's patience has persevered for so long that they have at last heard. If he closely examines his life, such a person will see that he owes his vocation to God's long suffering.
There is the possibility that the one who has remained single has done so because they have pushed the voice of the Lord calling them into the background, and hidden it away. They have, in effect, declined to answer their calling to a particular vocation. Adrienne von Speyr again:
The muting of the voice is such a frightening event and it so unsettles the person that whoever has said No or tried to put God off, postponing a decision, is a permanently marked man. He is and remains recognisable. He has pushed aside the experience of his life. In the future he remains embittered, dissatisfied, sarcastic and fault finding, and he never grows tired of ... trying to prove the impossibility of discipleship.... The No impresses itself and remains, and it is at times more capable of transforming a person's spiritual physiognomy more deeply that the Yes would have done.
Three of the chapters in Adrienne von Speyr's book are devoted in turn to the evangelical counsels of poverty, chastity and obedience. Writing in 1948 (so far as I can tell), Hans Urs von Balthasar began to present the life of the counsels as appropriate to the lay state of life in the Church. A work that he then originally entitled The Laity and the Religious State was later re-edited and published with the title The Laity and the Life of the Counsels. For both Adrienne von Speyr and Hans Urs von Balthasar, the form of the evangelical counsels, whether expressed in promises or in vows (private or public), is available to the lay person and represents the form of the Yes to the vocation from the Lord. An Apostolic Constitution of 1947, Provida Mater, gave lay associations in the Church the chance to achieve a canonical status as secular institutes. Hans Urs von Balthasar saw this as a great opportunity for the lay person - the secular institutes were a possibility open to the single lay person whose status in the Church otherwise remained anomalous. Writing of the potential of the lay person living the life of the counsels, Hans Urs von Balthasar says:
... the layman would never need to fear being unfaithful to his spiritual mission even in the most worldly turmoil of his professional work. For he would not have chosen this mission for himself; it would have been entrusted to him in the obedience of the counsels, and he would carry out under obedience all that it entails, even what appeared its most worldly dimension ....
It is scarcely possible to gauge a priori the springs of supernatural power that would be unleashed in such a life. Working in the same practice, office, or factory, but freed from the absorbing and often depressing concerns about family and earnings, the layman in the state of the counsels would have an incomparable advantage in terms of time, freedom, and productivity, not to mention the possibilities of taking up deeper questions that may not be immediately lucrative but are much more necessary, when seen with Christian eyes: public questions for which those with such busy private lives have no time, questions affecting the Church in the civil sphere, in which married people, who are dependent in so many ways, perhaps do ot want to take the risk of getting their fingers burned, questions of the apostolate in the lay milieu, which the priest finds impossible or at least difficult to approach ...
It is the spiritual value and significance for lay people of a life of the counsels that Hans Urs von Balthasar goes on to draw out, not just the practical possibilities of the single state. The future tense implicit in Hans Urs von Balthasar's words seems strange to us now, who so much take for granted the existence of lay movements and ecclesial communities in the Church. I have found it quite fascinating to see a kind of prophetic wisdom in the position of Adrienne von Speyr and Hans Urs von Balthasar, and it lies in this. Many of the new movements and ecclesial communities have, gradually, evolved a core group at the centre of their life who choose to live the life of the counsels. This is true of Communion and Liberation and of the Focolare, to name but two.
I would want to argue that the single state in the Church is not a vocational state in itself. It is full of opportunities to respond with a Yes to a vocation in the Church, the movements, ecclesial communities and secular institutes providing forms for that Yes in the single, lay state. Or, of course, it provides an openess to a vocation to the priesthood, religious life or marriage.
So, what of the person who reaches their later, or slightly later, years (or, to use a phrase now not often heard - the person who reaches their "middle youth") and is still single? Personally, I find the suggestion that being single in some way constitutes a vocation in itself patronising; it assumes a Yes that is not there in lived experience, and to try and pretend that it is there is unreal and, in the end, unhelpful. I suspect that it is possible that one is called to live, for a long period of time, in an openess towards a call that will only be revealed later in life; but that involves the radicalness of a living obedience, from one moment to the next moment, day in and day out, without the external scaffolding of formal commitment, that is a rare and quite exceptional, spiritual gift. One can also find a call to engagement in one or more particular activities on behalf of the Church or of the Christian mission in the world; but these engagements, providentially good and genuinely charismatic as they may be, remain a "secondary mission" that the Lord "tolerates". They cannot replace the "first" call that is pushed into the background in a refusal, a No.
UPDATE: This is a post that makes a couple of relevant points .... in its references to a person's "inability to commit to anything" at a certain age (!) and in its suggestion of inviting the girl out anyway instead of making an excuse ....