Yes, your body does change a lot as you get older. From 11 to 16 the body changes a LOT. Your hips will get wider (to accomidate a baby's passage through the birth canal) and everything will get looser. Thus, tampons will get easier to insert.
By age 35, a woman who has never had a child has generally the same shape as a woman who has. As the body ages, hormones change, and the body shape changes.
Personally, I use Instead Cups or the Diva Cup. I LOVE them. ON my heavy days I have to change a super plus tampon AN HOUR OR LESS! I have always been a heavy bleeder. These two cups when inserted will last 8-12 hours w/o having to be removed. It's great to be able to go out all day and not have to worry about finding a place to change my product. Plus they're super comfy. They take some getting used to, but once you figure them out they're great. I just change mine in the shower for the first few cycles to help me get used to it. I prefer the Diva cup b/c once bought, you never have to buy tampons or pads again. Home - The DivaCup, an innovative menstrual cup the best alternative to Tampons and Pads Menstrual Cups, Period Products, Feminine & Menstruation Products - Softcup