I'm 24 and have had reoccuring uti's since before I can remember (maybe around 5 or 6 years old) Around that time I had a surgery to lengthen my urethra and all was fine until maybe 6 years ago when they started back. They aren't due to sex, or that I don't drink enough water.
I most recently had one so bad it felt like it was affecting my kidneys but when I went to the hospital the doctor said my urine tested negative for any bacteria. My family doctor also recently mentioned that my urethra may not have fully developed as i've gotten older since I was so young when I had the surgery.
So just a thought to some of you, it may be caused by the length of your urethra.