How common is it that a 12-year-old is stronger than an adult female?

Well I’m just asking since I’m a 22 yo woman and I recently realized that play fighting with my much younger brother and his friends isn’t as fun as it used to be. I used to be able to handle them, but now they’re surprisingly strong and some of them can pull me down easily. That’s ok of course, but still I didn’t think they would be able to do that at that age and it stings a little. Silly I know, but I’m curious, is it simply me being weak?