We have taken our son since he was 4.5 months old so I think any age is a good age.
Maho beach seems to attract lots of families with kids. I suspect it is because of the ease of parking, the shallow water and the generally very calm water.
From the parking lot, Cinnamon will be a longer walk but there are facilities at the beach.
Are you thinking of camping?
For camping at those two spots those ages are good. The trick is that you are going to need to walk them to the bathroom. I started letting my daughter hit the bathroom by herself at Maho campground when she was 6.
There are a lot of kids that age at both locales. At Maho, we had the youngsters making their beach plans together.
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