I must be strange, I can't ever remember when I was in my late teens, early twenties ever looking forward to being married, having children etc, and never even contemplated being middle aged. I just lived for the moment.
I did get married at almost 25 but it wasn't planned and happened suddenly. The child I had at 36 wasn't something planned particularly either.
When I was middle aged I never considered old age either.
No point in expectations in advance in my opinion.