So, the processes of growing to maturity and senescence (how biological functions break down with age) are actually two different biological processes.
Maturation is largely driven by hormones released as the body grows and develops. It is actually just a continuation of the same process a person goes through as a fetus.
The things we associate with growing old, however, have to do with the physical degradation of our DNA.
So it makes sense that Superman would grow into an adult as normal. It also makes sense that, as long as he continues to absorb the radiation of a yellow sun, his biology would be able to repair the genetic damage that would lead to aging and death indefinitely, making him effectively immortal.
That said, the last yellow sun will die long before the heat death of the universe.
Just a bit of science that I’m sure at least Ethan will appreciate from a fellow physicist.