My husband's dad was 21 years older than his mom....his dad actually did look like his grandfather!
My dad is only five years older than my mom, but my mom has always looked so young, she and my brother and I often got mistaken for my dad's kids, like he was a single dad with three kids or something. In fact, when he first saw her, riding her bike around his block, he told his friend that she would be really good looking when she grew up. His friend then said she was in college with him! She was 19 and my dad thought she was around 12 or so. He started dating her shortly after. Lol!