I'm not sure that 'these days' are much different to the 'olden days'. I was a teen back in the 80's....and by the age of 18, I'd probably had about 4 partners.
Having said that, 4 partners by the age of 16 (either back then or now) is concerning. It would indicate to me that the girl very possibly has some self esteem issues or other emotional damage which she used sex as a salve for.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.