You can still ride together as long as you want, but the roles, ultimately, reverse.
I’m 41 and I regularly ride with my 75 year old Dad. The trails have got tamer – mainly roads, back lanes, gravel rather than MTB trails nowadays – but we’ve done some longer tours like Hadrian’s Wall, the Sandstone Way, John Muir Way and others. I’m now his domestique, it’s my job to deal with the logistics and pack the snacks, plot the GPS track so we can navigate from a Garmin rather than the guidebook on his bars, remind him to eat and drink regularly, and mainly be on the front to block the headwinds, and be the one to take the pictures so the trip’s recorded. With a few health issues that he’s had, I’m also there to keep an eye that he’s still upright and rolling, not keeled over in a ditch. We take it in turns to buy coffees and lunches, we chat a lot, and we generally have a good time. It doesn’t have to be a competition.