Apparently, Jack and Rebecca Pearson are really good at aging.
The finale script page, published by EW, reveals that a 28-year-old Jack first met Rebecca in 1972, at the insistence of the meddling but well-meaning Mrs. Peabody. We know from the show's pilot that the kids were born in 1980, making Jack (and presumably Rebecca?) 36 at the time. That makes sense; the show opened with the kids' 36th birthday, and this means it would have showed Jack's 36th birthday opposite that. If Rebecca were also 36, it's realistic that she had complications with her pregnancy, too, which sadly led to the loss of the couple's third triplet.
But if they were both 36 when their children were born, that would also mean Jack and Rebecca are in their early 50s when their kids are teenagers. And they definitely don't look that old in the flashbacks
This Is Us has gone to such efforts to age Mandy Moore as present-day Rebecca, so why is there virtually no age difference between Jack and Rebecca in 1980 versus when their kids are growing up? Rebecca still looks like she's in her mid-30s when the kids are teenagers. It's confusing, to say the least. It would also mean she was a lot older than we thought when her band went on tour.
This Is Us isn't the first show that's failed to depict characters' aging realistically. In its season three finale, How to Get Away With Murder left fans wondering why Charles Mahoney hasn't aged in a decade, if we're supposed to believe he's Wes' father. I get that it would be difficult to have so many different versions of Jack and Rebecca at various life stages — but This Is Us pays so much attention to detail that not showing the Pearsons' aging just feels off.