Thoughts on PhenoAge estimates?

I’m 34 (F) and got some thorough blood tests to prepare for pregnancy in the next year.

Somewhat surprising is that all my bio markers are either ‘optimal’ or ‘normal’.
One of the results was for PhenoAge, which says my biological age is 21. 😂 any thoughts on whether this is legit? I know it’s just an estimate but wonder what is taken into account to calculate ‘biological age’