J.F.
An older child absolutely can get it - my son was almost 4, older than the norm. And so can adults, because I caught it from my son when I was in my early 30s. Oddly enough, neither my son nor I had mouth sores. He only had blisters on his feet, and I only had them on my hands.
The good news is that it tends to pass pretty quickly. Hang in there!