I never really agreed with casting Diane Lane, who still looks fabulous for 53, & was ridiculously 48 when she got the part. Even Meg Ryan would be pushing it imho, but if they're at the point of aiming younger, that's probably where the line for me is. However, I mean now. 5 years ago, when they made Man of Steel, even Meg Ryan would've been too young. So maybe my line would've been someone like Denise Crosby then, who's about 5 years older than RyanMeg Ryan is three years older than Diane Lane.
Ideally, The Kents should be old enough to look more like Clark's grandparents imho. There's supposed to be a starkness about the discrepancy. So if you were going to start Clark out at just after Smallville, then the Kents should be no younger than mid-fifties, hence Jonathan having a not-to-surprising heart condition
It's downright silly that both Tony Stark & Bruce Banner would be around the same age as May Parker, but seeing how none of these people are blood relation to the heroes, they skate by. Hell, the next time they reboot all the DC heroes with younger people, they'll be casting Uma Thruman to play Martha Kent, & she's a year older than me