Aging well is a nice (I'd dare: polite) thing to say when meeting an older person who dind't change much since the last meeting (i.e. several years ago) or perhaps even looking better than last time.
The weird thing is that the literal translation to Portuguese, envelhecendo bem, seems to me to have the opposite meaning: that person got a lot older...
Is this true? I mean, if I say this to my friend's grandpa will they think I'm insulting him?
Are there alternative expressions in Portuguese?
Even though bem can sometimes mean muito, I cannot see envelhecendo bem being interpreted as envelhecendo muito. You’d have to say with a very special tone and a concerned look in your face to possibly convey that idea. Envelhecer bem is not a common phrase in Portuguese, I could not find it in this Corpus do Português, but Google search returns 60,000 hits, both Portuguese and Brazilian websites. The ones in the first page have the intended appreciative meaning.
So it would be ok to tell your friend’s grandpa he is envelhecendo bem. But there are informal, friendlier ways of saying it:
O senhor está cada vez mais novo.
O senhor é como o vinho do Porto, melhora com idade.
