At 50, a "bit over 10 years" is a creative way to say she's in her 30's. 10 years in general I agree, not a big deal. 60 yo dates a 50 yo, nbd.
But 50 vs 37/8/9 means she's in a completely different place in her life. Most likely (and the OP hasn't said otherwise) still wanting to start a family, and running out of time.
OP also mentions online dating with girls in their 20's. IDK about you, but when I did online dating I set my profile to an age range for potential matches. I didn't stray out of it because that would have been silly. I wasn't looking for dates but for a partner, and I'm firm believer that having more in common than not, is more helpful for long term success, including approximate age. (YMMV, annecdotes and exceptions can and do prove me wrong all the time)
Well, if the scenario is what you're painting -- a late 30 year old with a hunger, maybe desperation, to marry up and have kids -- then you're right, he needs to be very careful about that aspect.
As for the younger ages ... as you probably know, men tend to look for younger women, whereas women generally tend to look for men in their age group. When I'm engaged in my annual fruitless ritual of checking out an online dating site, I'll search for women down to 10 years younger (I'm 58). It's a vast wasteland out there, at least afaic, so I lose interest after a week and forget about it.
I'm not going to go younger. Other men sometimes encourage me to, but I'm just not interested in chasing younger women. It feels kind of pathetic to me. I'd find it socially and personally embarrassing, to be dating someone in their 20s. Everyone (including me and her) would know what the exchange is -- my resources for sex with her (which often isn't that great anyhow, more packaging/promise than real). Plus with younger women you can get the ideological craziness and the higher risks of false allegations, etc., that come with it. There is also of course the higher risk of pregnancies, "accidental" or otherwise. And the "baby rabies" you mentioned, in those 28-35 or so.
One of the advantages of being 58 is that sexual desire isn't in the driver's seat anymore. It's not that big of a motivator. If I want sex with a young woman, I'll just get a prostitute like other 50 year old guys do, lol. It's not worth all the time, trouble, expense, headache, risk, energy, and embarrassment of trying to date and establish a relationship with someone in their 20s. At least that's my view.
In an earlier post the OP said his partner doesn't want children.
In an earlier post the OP said the 20-somethings were seeking him out to pay their bills, and he shut them right down. He wasn't seeking them out to date.
Ah, good. Thanks for the clarification.