I had to google MASH. I graduated high school in 1987 and it was not a thing.
Also from 40 to 50 for people of decent wealth/income and indoor jobs is not going to be a big change. Given Kevin has celebrity bucks and likely skin care routines.... and likely some help from a dermatologist... he isn't going to age as much as a construction worker or farmer outdoors doing manual labor in the sun for 10 years who likely doesn't do a lot with skin care. Also Beth is of the income level that she would likely have a great skin care regimen and also a dermatologist to take care of things since she is very clearly into style, hair, makeup, etc. (not in a bad way - just that she is always stylish and we've seen her at the beauty salon)