18 to 25 is the most highly sought after market share for most products. For many reasons , including disposable income.
edit:
This shows that both the 18-25 and 35-44 age groups use smartphones about the same amount.
http://www.go-gulf.com/blog/smartphone/
What it doesn't show is that companies like Apple are selling way more than a phone, they are selling an ecosystem. If you get into that ecosystem early in life, you are much more likely to remain in that ecosystem. Apple would be quite foolish to 'not care about those 18 year olds' and I would be really surprised if that was the case.