How can I tell how old my turkeys are?

I bought 5 turkeys and would like to know how to tell their age. I would ask the people who sold them, but they have since moved and changed their number. They aren't fully mature yet, and they don't gobble, but they are pretty big (at least twice the size of our large rooster). I'm guessing they are around 3 months. How much would a 3 month old Broad Brested Bronze weigh?