Yep, the deposit is the same, $300 per person, no matter which trip you're talking about or how old the person is. Unrefundable after 14 days, balance due 90 days before the trip starts.
And you get 6 months with 0% interest if you put it on your Disney Visa! And you can use Disney Reward dollars towards the balance... Heck, you could probably use Disney Reward dollars towards the deposit...