@Saiddias - Far more 'serious' detail than needed for most questions on any manga site, let alone on this subject, but here it goes.
Legal I'd think, same 'real' age and mental age (well based on desc. Tbh haven't actually read it yet.) I'd think it'd be considered like those irl with various medical conditions that stop or slow physical development; regardless of physical looks they can be considered past age of majority/consent when they pass said age, unless their psychological development is effected in which case things generally get more complicated depending on the exact situation, extent of disability, location etc.
Having said that laws differ so much place to place irl, let alone in given fictional universes, so who really knows?
... Well I guess the author...
.... And maybe your local/Federal (or whatever is applicable) police/justice system/etc....