I started at 24-25, but it does come down to genetics. I am very fortunate with my bone structure and other naturally feminine features.
I know someone who is younger than me but much more masculine bone structure and unfortunately will need FFS.
You friend should be prepared for the possibility that FFS will be a requirement, it can be the case for anyone at any age (well, not in your case, but anyone after the age of 20ish when bone structure is fixed).
Having said that, I am convinced that younger the better, not so much because of the actual changes... but just being younger in general is easier I feel, socially and physically. But that's just how I feel, plenty have successfully transitioned socially and physically in their 50s too.