I'd stay 21 is very marginal. It fails the half the age+7 rule by a long shot (by 5 years, in Ioan and Bianca's case it would be one year) for one, but that's obviously not a established system and doesnt consider the fact that every human develops differently.
A 21 year old woman may be fully matured by that age (different story for men), in which case a age gap doesn't matter the slightest, but it's 50/50 at that age. If we were talking about a 25 year old I wouldn't have any concerns (nowadays many see even that critical which I disagree with)
Then there is also, and that was my point, that I don't believe that they started dating at 21, let alone met at 21. So you can cut some years there, and then it becomes critical because dating someone you met when they were underaged is definitely a different kettle of fish.
Now I wouldn't really care, but that's IMO a reason why he may run with this story that doesn't really make much sense (unless it's completely made up as some said to put Alice into the center of attention). Plus Tone has reposted pretty disgusting theories from Alice's socks that Ioan was having sex with 14 year old child actresses, so I think when people make these kind of accusations it's worth to scrutinize their own situation