I don't think it ever changes. It seems odd (but awesome, don't get me wrong) to me that we're discussing wedding plans and registries and combined incomes - at 28 and 26 we're definitely old enough to be making these changes and we're some of the last of our friends to do so - but it's a big step. Other friends have similar big life changes - mortgages and babies for a start. As we look to our parents' generation, we can see that they are having to make the adjustment to being grandparents and they also don't feel "old" enough to be the grandparents (although they made their parents grandparents at a similar age).
They say that you are only as old as you feel... and for me, that age differs. My students don't remember apartheid or a time when the internet didn't exist, that makes me feel old! Get me on a dance floor on a Saturday night and I feel sixteen again. Licking the filling of a cupcake and the pleasure I get from that experience makes me feel about three! I'm so excited to be moving forward in the story of my life but this new chapter will take some time to not feel so grown up!