When will I "fill out"?

Like you, I was very tall and thin at school and at college I filled out a bit, but around 25 is when I really filled out. I was 6ft at the end of school and weighed about 70kg... at 25, I was 6ft5 and 105kg with hardly any fat and still the same now, so sometimes it takes a while. It really depends on genes though, some people are naturally thin, some are tall, some are strong etc. Just have to wait and see!