Yeah, exactly.
My folks married when dad was 40 and mom was 20… THAT was terrible for everyone involved, especially since he was all about old-school traditional masculinity while she was an upstart young feminist go-getter (who earned more than he ever did, had a ton of social and business success vs his shut-in lifestyle, etc). There was so much fighting, screaming, cup-tossing, cheating, separate bedrooms, more dishware-throwing… ugh. Thankfully, I had friends’ families to model healthier relationship dynamics.
That marriage fell apart not a few years in, but they reluctantly, barely stuck together till I turned 18. I wish they hadn’t. Then they finally divorced and each remarried to someone closer to their age and personality, and both are much happier now (and still friends, somehow).
I suppose had they been 70 and 50, it would’ve been less dramatic, but 20 years is a pretty big gap however you spin it, crossing huge chasms in the societal culture at large and also technology (which often affects culture). Ten is much more reasonable.
Now that I’m his age when they married, I can’t imagine dating – much less starting a family with – some 20-year-old. Not only is it vaguely creepy, having a conversation with one feels like talking to a kid. But hey, maybe I wouldn’t feel so bad about my lack of retirement planning 
My own GF is barely a couple years younger, and we share the same values, similar backgrounds, etc. Her parents also had a rough marital history. As a consequence we both try to avoid that in our own relationship, instead trying to communicate, cooperate, and compromise well. We don’t really ever fight about anything, just talk things through calmly and fairly and work out a mutually beneficial solution.