Her hair sounds so much like one of my granddaughter's, it was so fine, even wispy, and wouldn't stay in a a pony or pigtails or braids, even plastered with gel or sprays, just slipped out. (And everyone else in our family has thick hair from the get-go!)
I did start trimming it when she was about 2.5 to 3, about three inches the first time, and kept the trimming up for years, but it truly didn't begin to really thicken up until she was about 6.5 or so. Since ponies or braids were a complete wash I compromised and kept her bangs trimmed and the rest down (used lots of headbands) to keep focus on the front, then when she was around 7 or 8 she decided she wanted all her hair to be one length. Now that she is almost 11 her hair is considerably thicker than it once was, and a little past the middle of her back, but really is not what you would call thick. It's comparable in thickness (and color, which is why I thought of her) as Cameron Diaz's.
Personally, I didn't see that trimming ever helped it to thicken, she's never lost the fineness though it isn't baby fine, it just kept it neater.