Britney's main fanbase is people in mid to late 20s. These people were the girls and boys who were voting for her on TRL and waiting outside Times Square whenever she made appearances on TRL. I didn't see too many young teens at her concert when I went in 2011.
At my age, she was marrying everyone she met recovering from a knee injury. It was just from a music video and she swears she'll be back to normal soon.