I notice that most volunteers are young; I am retired.

Hello! not exactly a retiree here though i am a mid-career finance professional in my early 30s. I first started volunteering overseas when I turned 30 (probably considered a late-starter) and there were indeed lots of volunteers still studying or just graduated from university, most of them are 18 to 24 year olds. It did take me a while to get over the fact that I'm at least a decade older than these kids, and i also thought there might be a communication gap - but i think it boils down to individual personalities, open-mindedness and your willingness to widen your social circle. I was fortunate enough to meet the friendliest, warmest and energetic young people ever, which definitely gave me the breath of fresh air i needed outside of my mostly 30-40s only peers and co-workers. we had enjoyable, meaningful conversations about life and everything in between. Talking to these young people definitely makes you give them more credit than they deserve. after all, most of them have traveled out of their comfort zone and open to new and different cultures/experiences!
Conversely, as they have yet to experience the real working world, many of them will be curious (maybe even anxious about their future prospects and the economy) and here's where you can offer your wealth of life experience and advice. Trust me, they genuinely would want to know and definitely appreciate any life lessons. If you embark on a program with a mindset that everyone has something to offer, you'll get so much more than you ever expect. two years ago in cambodia, i met a 25 year old australian girl who came with her mom, a retired primary school english teacher. from what i could tell she had an amazing time and definitely didnt have any issues fitting in with other volunteers! Also, you always have 30-somethings like me (there are a handful) so i'm sure you will feel comfortable whichever program you choose. Good luck and don't hesitate, just go for it!