According to this source, there were an estimated 3.6 million EU nationals in the UK in 2016, of whom 2.4 million were in work. The BBC Suggests 2.9m and 2.15m. That gives between 0.75 and 1.5m not working - assuming roughly half of these are under 18, we end up with between 2.5 and 3.15 million people over 18.
ONS figures give a total population of 65.6m, of whom 81% are over 16*, so roughly 53m people of voting age.
Therefore it's roughly 5-6% of the population, depending which figures you believe...
Caveat: I don't know if those figures include people who have dual nationality, or Irish, Cypriot or Maltese citizenship, so it will probably be lower than that.
*As pointed out below, voting age is 18, but those ONS statistics don't break down 16-18 year olds.