I have one; he's 6. There is one more at primary school, but older, and a couple more at the local secondary. We live in East Anglia. We also have a friends with a Theo but that's because great minds think alike! I love the fact that it's short, but still a 'grown-up' name. I don't really like Theodore, which is too much for me.
Be warned, though - in my experience all Theos are hugely spirited and rather naughty!!