To begin with, everything is relative.
At 16, people in my 30s already seemed very mature to me
At 20 I was sure that after 40 I no longer have a normal life.

But ... for a second, I'm 26 years old now and I'm happy in my age and body.

So what I want to say, actually ...

At 20, your appearance is not your merit. A mix of nature and genetics. It will be logical to respond to a compliment about your appearance: "..thanks to your parents."))

After 30, 40 years, etc. - you already look in accordance with your lifestyle (every year this correspondence is more and more) and as much as you invest in yourself

Any changes in the body, face, style, hairstyle, etc. are about YOU, your investments, efforts and your merits.

Old age cannot be undone. But it can be very different. And this largely depends on US.

What time does old age begin for you?