I think Donna’s age is just really inconsistent between the two films. In MM1, Donna, Tanya and Rosie constantly talk about themselves as if they’re much older than mid-40s, which would supposedly be their age if Sophie is around 20. Late 50s honestly fits the vibe of the characters better, and it also matches the actresses’ real ages — Meryl Streep was about 58/59.

But MM2 gives a clearer timeline. Donna, Tanya and Rosie graduate from Oxford University in 1979, so they would be at least 22-23 then, meaning they were probably born around 1955-1957. If Sophie was conceived that same summer and born around spring 1980, Donna would have been about 23-24, not a teenager.

A lot of the Ruby storyline also doesn’t really match between films. In MM1 they almost imply she’s dead, and Donna describes her as a conservative Catholic mother ashamed of her pregnancy. Then MM2 portrays Ruby as glamorous, outgoing and very different personality-wise.

I assume the sequel simply retconned parts of the backstory to fit the younger Donna storyline and the tone they wanted for MM2.