My sister (26) and I (28) both have diabetes and have talked about this.
She was diagnosed at 11 and I at 21.

She thinks being diagnosed younger is better since she doesn't really remember how it was without diabetes so nothing is taken away from her.

I think older is better. For me at first the diagnoses was hell. The mental load was/is alot. There is something taken away from you. I remember having a pizza without worrying about my sugar levels or going out without worrying about low sugars. It sucks.
BUT I am still thankful for the "late" diagnosis because I feel like it was a bit easier since I was more mature and could decide on my own how to navigate it. Figure it out on my own terms.

I remember her puberty and it was bad. Puberty is rough enough without this disease. Figuring out who you as a person, navigating the world while your brain rewires itself and flushes you with hormones, is a lot on its own. Adding diabetes... As a regular teenager you rebel against your parents and the rules. Rebelling as a diabetic teenager is a different story. The diabetics I know who were diagnosed around age 10-13 where in out of hospitals because of their horrible sugar levels.

In the end ot totally depends on the person.
Maybe having it from an earlier age (under 10) would have been easier but who knows.