At 31, I feel like I am just now leaving the 'young talent' group, but not yet considered older. Standing in the middle, kinda, I think I can see both sides. To help deal with the agism issue, my hypothesis is that if you can zero in on the exact misconceptions they might have about you, or older programmers in general, and demonstrate that you don't fit that mold, you're in.
Some younger engineers will worry if older engineers lost their hunger for learning new technologies. In their position, they are well aware of what is fashionable, or even just current best practice. This is mostly because as new people they go straight to the new stuff. For example, new programmers don't learn SVN, they go straight to git, and github is a way of life.
So, right or wrong, it is a red flag to them if they see or hear about people advocating what they consider as outdated approaches.
But, importantly, they also read books by the folks who are 20-30 years older, but never lost that drive. They go to talks by these people and take notes, buy their screencasts, and brag about what they learned. They want luminaries to guide them. So, maybe, establishing expertise over what they consider important is a way around this misconception the youth has.
I imagine you could send positive signals by fixing or improving some popular open source project. Maybe write some interesting toy code, a game, or something that solves a development problem, and add that to a personal site or your github account. Any way to demonstrate what someone with 40 years experience can do, in a context they can see and interact with should do the trick.