Yes, Ford is getting on, but what they can do with makeup and image trickery is amazing these days.
Indy Jones was always meant to be a rough, craggy, worn character anyway, and who the hell else could play Indiana Jones??? It just wouldn't be the same without Ford.
The new movie (in a timeline) will obviously have to be set after Indy 3, so there may not even be Nazi's in it (set after the war?), so they may well get away with it. Unless they are planning Indy 5,6 etc, there is no point bringing in a new 'replacement' as a sidekick. Anyway, once Ford finally hangs up his hat and whip, there is no-one who could really fill his shoes. He IS the only Indiana!