Both of Bezos' wives are 6 years younger than him (he's 62 and both MacKenzie Scott and Lauren Sánchez are 56). Their ages are only noteworthy for being so completely not noteworthy for a rich guy. It seems like he has age-appropriate taste in women.
I think the implication was older than 18.
That could make sense. I assumed it was a dig at how his current wife has had a lot of bad work done.