This is actually the age of the oldest observable galaxy. The universe is a little older than 13.7 billion years (yes US billions), 13.7 is just like a minimal figure. We know for a fact the universe is at least 13.7 billion years old. 15 billion was the old value and most scientists think the universe is a little younger than that. It depending on where you look / what method is used but it's something between like 13.9 and 14.1 billion years (the "textbook value" they teach in public schools and universities is rounded off to 14 billion), but could be anywhere between 13.7 and 20 billion years old (though most scientists are pretty sure it's less than 15).