Your post reads like I could've written it. I'm over 50, have only been cycling for about 3 years, and do group ride with a lot of younger guys. I have the same issue wiht hills when riding with the experienced groups .I can always get up the hills at my own pace, but not at the faster pace of the group. Here's a couple of things I've found helpful: Try to stay near the front of the group (or at least not at the back) as much as possible. Adjusting to all the yo-yoing and changes of pace that are amplified at the back is exhausting over a long ride. If you do get dropped on a hilll, get back with the group as soon as possible, even if it means an all out sprint to catch back on. I've found spending the effort on the sprint to be less tiring than riding solo for a longer period. Stay close to the wheel ahead of you... more than a few feet back and you've lost most of the benefit of drafting. On the hills, I find that I am more successful if I go a little stronger (faster) at the beginning of the climb and then settle in to a pace I can maintain. This isn't always possible from the back of a large group since the group changes pace differently, often slowing more than would prefer initially. I try to either get near the front, or to the inside as we approach a hill so I can do my thing and not worry about adjusting to the rest.
Anyway, that's about all I've figured out so far, other than continuing to ride hills and work to get stronger!