I thought it was 18. I was irked a bit one time there were some young kids in Solarium and in hot tubs.
If it's 16, that's good as my daughter is 16 and I thought she wouldn't be allowed in there. Either way, we'll check when we go.
Last cruise, the solarium pool was very small and adults were just standing in the pool socializing (which they're allowed to do :)), so I went to the outside pool.