Tiffani Thiessen -- Good Genes Or Good Docs?

Tiffani Thiessen put her aging on a "time out!"

Here’s a 15-year-old version of Bayside High’s most popular girl at a "Saved by the Bell" photo shoot back in 1989 (left) and 26 years later ... the hot mother of two -- who celebrated her 42nd birthday earlier this week -- at a charity event in Los Angeles last month (right).

"It's alllll right!"

The question is…