I know a lady, not exactly young, who could not manage her solo at the 'usual' place and seemed to stick there. Her difficulty was only in the 'rounding out' and there were no problems with her general flying or circuit work. Change of flying school and they took the attitude of leaving it 'til later but continue with instruction. She has now completed most of the course including navigation work and is coming up to 80 hours, still hasn't gone solo (although this is now more weather related than her ability). Point is that she is a really good flyer and loves every minute of her flying. Theoretically she should have been 'dropped' a long time ago but in practice she is having a great time.
I am also aware of a very elderly man who will never get to fly on his own - and he knows it - but he still has his regular 'lessons'.
I learned with my own flying that you have to forget about targets, it's not a competition - it's about personal fulfillment and enjoyment.