They're large, generally humanoid robots. But it's not out of the ordinary to have alternative forms, either as backup or to switch tactics in battle.

[What else...]

Obviously because of the cost and maintenance, the military and other organisations can afford to keep them. ['Other organisations' being secret ones, terrorists and anything else.]

Most of the mecha series I've seen usually have young pilots as the main characters, because of the target audience's general age range.