Am I being U to even wonder why this matters?
People age differently. I've noticed that some people age very quickly, and the attitude that an arbitrary age somehow isn't "young" but should be referred to as something else, or you should act in some more middle aged way, is one of the most surefire ways of ageing quickly. I think it also correlates fairly closely with poorer health expectations and life expectancy.
The classic example would be the woman who gets pregnant at 15 and by the age of 25 has 3 children and lives quite a hard life keeping all that together. Compared to someone who goes to university for 4 years, has a gap year or 2 working abroad and having lots of different experiences, then goes back to university and does a Masters, then finds a graduate job along with lots of other graduates and buys her first flat, and eventually meets the serious boyfriend with whom she will eventually marry and have children with. The former is going to age much more quickly than the latter and have experiences in life that may make her more cynical.
I also think you'd need pretty ill health not to still be young at 35 (or whatever).
Some people are also obsessed by age. I have a couple of lovely friends who literally cannot have a single conversation without mentioning their age in it somewhere, as in "now that I'm old". When in actual fact both are fit, active and in full health and still in their prime of life and in no way showing signs of becoming old. I think its particularly bad in Britain for making women over a certain age feel almost guilty for not adopting a pinafore and headscarf and hiding away. Other countries have noticably more women over 55 reading the news and not being pensioned off. Its very noticable.