I checked the average height for a twelve and a half year old boy and it turns out I'm in the fiftieth percentile. The weird thing is that eight months ago when I got checked, it said that at fifty-six inches I was in the fifteenth percentile. How is it possible that I've grown four inches in less than a year because I haven't noticed much of a change? I don't know what my growth chart says because I don't know how much an inch is. I'm still rather short compared to the girls, but I'm pretty sure I'll catch up.
Is it normal for twelve year old boys to have hair under their arms because I now have a patch on each arm but it's not thick. None of my friends seem to have any and nor does my twin brother.
A lot of people don't notice changes in height when it is happening to them. We have a tendency to compare ourselves with others, but if everyone else is growing then our "ruler" is changing along with us. You were smaller than average, but you've hit your growth spurt slightly on the early side so you are now catching up with other boys who will get their growth spurt later. The hair and the growth means you are in stage 3 of development.
If you and your brother are fraternal twins, then the timing for development will not be necessarily close for the two of you. Even identical twins can different by several months.