I'm 65, and I often hike with fairly large groups (I help lead beginner classes); they tend to break into smaller groups of folks along the trail, who have similar hiking speeds. I often am with the slower groups, partly because someone among the leadership group needs to, but mostly because that's my comfortable hiking pace, too.

If you can find one or two other "slow" hikers, form your own sub-group. Just be sure you have a good map, very specific information on the route and campsite of the rest of the group, and are self-sufficent enough in gear that you can spend the night apart from the larger group. Also, be sure to let the group leader know you're going to be lagging behind; if he/she is any good, they'll know how to accomodate your pace without slowing down the group (often by giving you the specific information noted above, and by occasionally having someone else wait to be sure you're still progressing, or waiting for you to catch up at stream crossings or trail junctions.

Once you've made those kinds of arrangements, relax and hike at your own pace. Slow only matters if you're trying to win a race.