i think its ok if ur comfortable with it, but you have to be 16 to give consent otherwise its illegal (under the sexual offences laws) but other than that it should be fine (thats what i remember from my PSHE lessons anyways)
why would a 19 yo want to pursue someone who's 15 19 yo is sus to say the least
maybe its the otherway around lol but i mean everyone has their preferences personally, for me, the age gap is too big at that age, cos of maturity levels e.g. 15 is doing ur gcses but my friends brother is 19 and he's getting married this sunday