viltrumite aging slows as you get older. we don’t know exactly when that kicks in but definitely older than 18. plus remember that mark got his powers late

Their aging slows as they get older, otherwise, they would remain adolescents for hundreds of years.

there’s 2 types of long lived growth in fiction

proportional progression where the growth rate is the same as a normal human but on a longer timespan

and the fast maturity progression where they age like a normal human until somewhere in their 20-30s before the aging slows down dramatically

viltrumites are the latter

I think Viltrumites slowly stops aging when reaching what would be human’s adulthood.

The show explains this.