It's not a matter of age, or sexual experience. It's a personal preference of what's more comfortable. U by Kotex is a brand designed for tweens. The tampons are a little smaller. Try that. Contrary to popular belief, sex doesn't change the size of your vagina, unless your partner has some kind of seriously massive equipment, because the vagina is a muscle. It can expand and contract. Aging, general growth, and puberty can effect size though. You can use a tampon whenever you fell like giving it a shot
You can wear a tampon as soon as you start getting periods, if you prefer it. Some women wait until they are no longer a virgin as they find insertion easier. If you are a virgin and want to try tampons, try them during the flow, and use a smallest size tampon (some brands call them "Junior" size.)