Twenty-two.
Honestly, I don't think age is a huge game changer here. We're a little more likely to hop at bad deals because we get excited more easily and we're more likely to think we're invincible, and we don't have as much life experience, but these aren't insurmountable obstacles. It just comes down to looking at the numbers and listening to what they tell you, no matter who excited you are, and to not let people bully you just because you're younger.
There are undoubtedly different advantages and pitfalls to becoming a landlord later in life, so I don't think it's worse to become a landlord younger. It might be a weird thing to do in your social circle, but don't let that stop you. If it's what you want to do, or if it's your strategy to FIRE, go for it!