I think that roughly, we compare time as we experience it with time as we have experienced it.
So when I'm 10, another 5% of my life would be 6 months.
When I'm 50, 5% of my life would be 2½ years.
I suggest that 6 months when I'm 10 would feel roughly the same as 2½ years when I'm 50.
This would explain:
- why time seems to accelerate as we get older
- why we can't remember much about when we're very young
- why people tend to get more patient as they get older - the waiting time seems less
An interesting consequence is this. I'm 29. If I try and relate to an 18 year old (for example), from my point of view the age difference is 38% of my age. But from their point of view, the age difference is 61% of their age. Hence I'd think it was easier to relate to them, on average, than they would to me. This also fits with experience.