Is it wierd for a 19 year old women to be with a 16 year old?

Heres the thing my friend is into my brothers friend and hes only 16 and hes into her but in fairness their all actually fairly mature, they hang around wit people older than them, aswel as their own age em i dont see a problem with it but would just lke to know odrs opinions oh and its illeal to have at sixteen over here. im jus abit curious to see what people think!!