I started feeling old when I turned 18 because I was leagally an adult, but I was wrong. I really started feeling old when I was 20, but how wrong I was there... When I turned 22 and I saw al my friends graduating (except for me), I felt old (and lonely). Then at 25 I thougth "I'm a quarter of a century old" and I felt old. Now I'm about to turn 26, so I thought "I'm more than a quarter of a century old" and I'm feeling old .
Jokes aside, I like to think we all have 3 inner selves: A baby, who expresses vital needs (hunger, cold, sleep, etc). A child, who expresses emotional needs (attention, love, hate, boredoom, etc.). And finally an adult, who expresses more complex social skills (faking feelings, responsibility, rationality, emotional intelligence, etc.).
Since the baby doesn't even know the concept of being young or old, I focus on the other two:
The more I feel identified with the child, the younger I feel: truly feeling the company of friends, embracing the experience of love without control, letting curiosity lead you, being able to stay in the moment; playing, joking, laughing and even hating.
The more I feel identified with the adult, the older I feel: schedule tasks, work, social compromises you don't really enjoy, responsibilities, money management, etc...
I think watching mlp makes me feel like a child before adult things strike back again; and given the over-rationalization I'm doing here, you might guess how old I feel.
But this is just an absurd model I've built in my head to make me think I have control over my own life .