we will in fact never know exactly what proportion of each age group voted. Some quick background: at General Elections, representatives from political parties stand outside polling stations asking for your voting ID number, and collate this information country-wide to figure out who voted (and guess how, based on canvassing data). However, they tend not to at one-off votes, such as referendums, and didn't on Thursday.
...
Sky isn't claiming this is collected data - it's projected, and a subsequent tweet said it was based on "9+/10 certainty to vote, usually/always votes, voted/ineligible at GE2015". I've asked for more information on what this means, but for now it's enough to say it's nothing more than a guess.