You just gotta go in and find out. After I've gotten a decent ways into the conversation, and I think this girl could be underage, I ask her age. If she's under 18, I politely find a way to end the conversation.
If she's on the older side, I usually wait for her to ask me my age. Sometimes you'll have to deal with an age objection on this, depends how old you are.
But regardless, go out there and approach.
I think the bigger issue is this.
Little nerve wrecking when it comes to approaching women during the day.
Approach anxiety.
The more approaches you do, the more you will realize that the issues with approaching that you think are problems aren't really problems but more so your anxieties. This at least is my experience, you could have something else.
Hope this helps,
BHL