With my oldies, I don't tend to notice real signs of old age until they're 2 or so.
One thing I think is important is hygeine of old lads. If they're old and a bit frail, they can have issues washing themselves quite as thoroughly, particularly around the naughty bits! When I have old doddery boys, I do regularly check their willies to make sure they're clean and they don't have plugs
With my really old lads, I would bath them semi regularly too, just to keep them cleaner. I had a very old lad recently who had complete HLD and couldn't get about well, and I had to bath him once a day otherwise he just got soaked in wee and it can burn their skin if left.
With rats with poor hind leg usage, its a good idea to keep nails trimmed as they won't be wearing them down as they normally would. For some reason, old man rat nails seem to be thick and grotty, like old man human nails
But I haven't really noticed these issues in any but the most ancient of rats, sort of 27, 28 months. All of my 18 month old lads are still pretty sprightly and well, and don't need much extra care from me.