Il n’est plus aussi jeune qu’il ne l’était.

In this sentence ne … plus expresses “not anymore” (it’s the most natural translation for the now), and aussi … que expresses “as … as”.

In the last part of the sentence, ne is a ne explétif (i.e. not a real negation) which is commonly found in comparison bases (this particular expletive ne is considered mandatory by most). And le stands for the previous attribution “jeune”.

The suggested “reverse” sentence lacks the above-mentioned ne and le, but sounds idiomatic otherwise: Il était plus jeune à cette époque qu’il ne l’est maintenant.