I, Karunaguin think that the age requirements for builder need to be lessend. I can just barely apply for mod and being 16 for builder is way to old. I believe you need to lessen the age restrictions a little.
I'm just guessing but I think the age requirement for builder is so high because builders get access to creative mode and can easily spawn in items for personal use. They probably want a more mature person so the chance is lessened and the easiest way to do this is to limit by age.
Feel free to take my opinion with a grain of salt here, but builder is an important role and to be a builder you need to be reliably active and trustworthy because of the perks you get -- two traits more commonly found among older players. Also, maturity is another thing that comes with age and is important.