I am not sure, perhaps "Libertas quae sera tamen respexit inertem" from Virgil. But wait, someone might have better options. After all Cicero wrote a whole book on senility.
Otium is a good word, but retirement has a ring of old age to it while otium is something you can enjoy all your life. So it would need some context.
If I wished somebody a happy retirement (because of old age), I would say something like ultima parte vitae in otio fruaris
or negotiis (publicis) remotus annosam aetatem iocundissime peragas
I suppose you need something shorter if you want to write it on a sundial
Maybe just otio fruere senectutis
Many thanks to you all for your replies, really like otio fruere senectutis - he is only 57 (lucky man!) I know he will google translate and it comes up with 'enjoy retirement age'