Sinner being a later bloomer and Alcaraz being a later bloomer doesn't mean Alcaraz is done developing or is in his prime yet. Yes Sinner has improved a lot since 2022 because at the end of 2022 he was ranked 15th in the world and not even in the discussion for best player in the world.
I would argue the difference between Sinner from 2024 to 2025 is basically about the same, whereas Alcaraz's improvement from last year is much more significant. ( 2024 he had 2 slams, 4 titles) This season 2 slams, 8 titles including 3 masters, and titles on all surfaces( clay, grass, indoor hard, outdoor hard)
Also even in 2022 when Alcaraz was the #1 player in the world he finished the season with a record of 57-13 whereas this season he finished the year #1 with a record of 71-9, in a season where he had a much better Sinner to compete against.
I think Alcaraz is maybe a year or so away from his prime whereas I think Sinner is already in his prime.
Just because Federer had won a slam at 19, didn't mean he was in his prime yet, his prime was probably around age 25. Same with Nadal who won slams while aged 19, but his prime was probably 2010 when he was 24.