Oh don't get me wrong, I definitely agree it's dishonest and unethical of them to not be upfront about their costs. The process they make you go through is incredibly time consuming.
Frankly, I'm kind of surprised that they -would- do something like this, given their reputation, and I'm confident there must've been some sort of miscommunication. I won't make any assumptions though, until we get to hear from the other side of things.
They've also built a reputation of producing quality students, and a network of employers associated with that. 18K is a fairly low price to pay for this, when you compare App Academy to the cost and time commitment of a university / community college program.
As for the tuition difference with age, I presume that a higher percentage of 22 year olds and under pursue a start-up job, where salaries are lower. Hence, they make up for the difference by charging an amount upfront.