Jennifer Love Hewitt: Good Genes or Good Docs?!

Jennifer Love Hewitt's looks have stuck since "Last Summer."

Here's an 18-year-old version of the "I Know What You Did Last Summer" star at an awards event in Hollywood back in 1997 (left) and nearly 18 years later ... the "Criminal Minds" actress -- who is celebrating her 36th birthday this weekend -- in Los Angeles last year (right).