By waiting until she’s an adult. A bodyguard that looks like a seventeen year old girl is perfectly plausible. There are a lot of reasons why you might want to stick a protection detail on someone that doesn’t look like a swarm of shaved gorillas in suits. But, the skillset, and the general maturity you need for a good bodyguard just aren’t things a teenager will have had the time to acquire. Sorry.
If they’re not actually a bodyguard, and it’s an ad-hoc, “I can keep you safe”, kind of situation, then that’s not an issue at all. They may even think of themselves as a bodyguard. That’s perfectly reasonable, and you have a lot of latitude on what is, or isn’t, a convincing outlook, because your character is setting the range for their own behavior.
But, keep in mind, they wouldn’t actually be a bodyguard, so, if their protectee is someone who would need a real security detail, then they’d be shut out.
If the protectee is someone who’d fall under the protection of the Secret Service or the DSS, then your character wouldn’t even be allowed inside the security envelope, unless there was some specific reason. Such as a close, longstanding personal friendship, or if they’re an immediate family member. Even then, there’s no way they’d be part of the security detail. In fact, if they were an immediate relation to someone under either Secret Service or DSS protection, they’d be protected by members of the same agency.
Again, the FAQ on FBI.gov will give you a good idea of the requirements for a Federal Agent. Since 2007 or 2008, former Presidents can opt out of permanent Secret Service protection, though, if they do, they’re required to maintain their own security detail.
I’m bringing up the Secret Service and DSS because they’re the most likely to employ people who look like teenagers. But, the people they’re hiring are going to have Bachelor’s degrees, and (usually) a history in law enforcement or the military.
Most major metropolitan police departments will have a VIP protection squad, though, the name will vary. A lot of times these aren’t dedicated units. I’m aware of one case where the anti-gang taskforce, the VIP protection team, and the vice squad were actually the same set of officers.
Corporations that hire bodyguards for their executives, draw from PMCs or security companies that provide bodyguard services. Lower ranking corporate officers might hire bodyguards of their own. This is somewhat more common in developing countries. But, in these cases, the shaved gorillas in suits, are more likely to appeal. Depending on the PMC or security firm, their personnel will also skew for ex-cops and ex-military, with some mercenaries, and depending on how rigorous a company is, some “ex-special forces” wannabies.
As a general rule, ex-cops make for really good bodyguards, the rest less so. The police skillset transitions into bodyguard work very well. Ex-military bodyguards can usually get the job done, and in rougher countries, they can be preferable, but they’re just not trained for the specific kind of threat assessment bodyguards need.
I’d recommend the 2004 version of Man on Fire with Denzel Washington, and any episode of the West Wing involving the Secret Service Agents (there’s a lot of them.) Particularly the episodes with Jorja Fox as Agent Gina Toscano.
I’ve trashed it before, but Taken does show a good martial form for a bodyguard, even if it’s egregiously out of place for Liam Neeson’s character.