OP, the only tours I know of that have age restrictions have them specifically to exclude geezers like you.
They're pitched to the 20-something crowd, and don't allow participants over 30. I've never heard of a tour aimed at the adult crowd that has age restrictions.
What you CAN do, though, is to pick tours that exclude the following groups:
1. The "moves at the pace of an arthritic snail" group
2. The "shop 'till you drop!" crowd
3. The "party DOWN!" group
Look for tours that emphasize lots of walking (or other athletic activities), don't have shopping stops, and are clearly aimed at either real nature enthusiasts or serious cultural/arts lovers, and you'll mostly have excluded the above groups. On the European tours I've taken with Rick Steves, we've had 70 year olds along - but since the tours require people to walk as much as 8 miles a day and be on their feet for up to four hours at a time, those 70 year olds were not in the wheelchair/walker crowd. And since the tour emphasizes real sightseeing and cultural events, there's no emphasis on shopping. Likewise, the party crowd, the shoppers, and the seriously out-of-shape were nowhere to be found on my Grand Canyon rafting trip.
Expect any tour that's actually worth going on to be pricey. In general, you get what you pay for when it comes to tours.