If the horse is well behaved under saddle and has good ground manners, which to me means that he stands tied, he respects your space, he leads respectfully, he yields to pressure in all directions, he doesn't nip, kick, try to run over you or anything like that, then I wouldn't bother boring him with "daily exercises." I hardly do groundwork any more with my 17 year old schoolmaster (I mean focused exercises. One could easily point out that leading the horse from pasture to stall while making sure it happens in a respectful manner is in fact groundwork) that I've trained and owned for over 11 years. If she takes it into her head to try being a bit pushy, she gets the three and a half minute refresher course, which usually means she has to give me a prompt "back" or turn in a circle and then turn on her haunches away from me, and that does the job. It's rare indeed when she needs such a reminder. She knows the score.
If your 15 year old isn't giving you any trouble, then daily exercises don't seem necessary to me. With any horse, I'd establish a basis of natural horsemanshippy groundwork, which really means learning how to communicate with it, mainly through pressure and release, and show it that you're the herd leader. But once these things are worked out, you only need to remind the horse of them in subtle ways, just maintaining your leadership, as you go about your normal routine of catching, grooming, tacking up, and riding.