How can I say idiomatically in classical Latin that someone is old but still looks young? Should I say bene senuit ("he has aged well"), should I use a participle like bene reservatus/retentatus, or should I maybe use the verb perennare? By idiomatic classical Latin I do not mean that it has to be necessarily attested as such. Well justified unattested answers are very much welcome.
If immortal is good enough for old, but still young-looking, you can refer to Virgil, who wrote
iam senior, sed cruda deo viridisque senectus (Verg. Aen. 6, 304)
The god in question is Charon, whose old age is fresh and green. This verse is very famous. Tacitus cited it:
cruda ac viridis senectus (Tac. Agr. 29)
You asked about classical Latin, but it is interesting to note that, doubtless because of Virgil and Tacitus, senectus viridis became a set expression in later periods:
At tibi senectus / Sit semper viridis (Marco Antonio Flaminio, 1498 - 1550, Carmina)
So, you can say adhuc viridis senex.