It's because of several reasons that M40 and even 45+ 50+ tend to outperform the younger AGs:

1. As mentioned, the true speedsters in the <M40 group typically race pro/elite in big races, and often don't show up in the AG standings. If it's a true national-class field counting pros-elites with the AGers, the top younger folks will definitely beat the top older folks as expected.

2. It takes time to learn to swim/bike/run to capacity. I think it takes about 5-7 years for even a 'serious' AGer to really get it all together, so probably longer for the early dabblers. I've def beaten the large majority of <30 AGers in triathlon that I race in local events, and the ones that beat me tend to be going fast enough that they're going to be racing elite the following year, and it's not because I'm so great, but because as a M45er, I don't get killed in one of the S/B/R disciplines like most of the youngsters do (most of them either can't swim at all, or swim so well from competitive swimming that they run/bike poorly.)

3. Kids and job responsibilities absolutely kill the 'normal' AGers under 45 years of age. I've been experiencing this firsthand as a triathlete from age 30 to my mid40s now. At 35-40, my young kid required tons of attention and time, pretty much sucking up nearly all of my time. And my job required more time as well, as that's the peak time for job advancement and learning new skills to solidify a better position. Once I turned 40-45 and my kid was 5-9 yrs old, things got a lot better timewise every year. Now my kid is approaching teenager years, and it's frankly shocking to me how much time I have on the weekend to at least do stuff on my own in the house - as great as I love spending time with my kid, I'm literally dreaming of the day that she's old enough as a teenager that I can actually leave the house and do an outdoor bike ride for 3-4 hrs in the middle of the day. I'd say my time availability to train now at age 45ish is literally 3-4x what it was when I was 37, mainly due to the kid and less so but also my job. It's becoming really, really clear to me why in my local 5k, I've won the M30-35, M35-40, M40-45 AGs nearly every year, but I've been beaten by different M50 guys nearly every year - there is just soooo much more time to train as the kids are teenagers+.

4. Last note - don't hope to wait until you're an oldster to do some great training! As coach Matt Dixon says, be your best self NOW. This doesn't mean you're going to sacrifice family and life to shoot for some KQ pipe dream, but to make the most of what time and resources you realistically have available NOW. The reason being - once you're over 40, weird but significant health issues absolutely start limiting you. A big one is early arthritis - a little niggle in your knee suddenly becomes a complete limiter for running fast or long. Other health issues become more and more frequent. So don't take your health for granted - triathlon training is one of the first things to go once a health limiter pops up, so enjoy it NOW.