Pretty sure you can get mono at any age. From what I've read, you're right, the doc can't do anything except diagnose it. DO be very careful if you think your spleen is swollen.
Because I've had it before and its the only thing my symptoms fit. Plus I'm pretty sure my spleen is swollen.
I'm not sure about being 27 as too old. I had mono twice while in college. I was supposed to be chilling out and relaxing but instead I continued to keep late hours and drank beer. When I was re-tested to see how I was doing, the results were worse! My parents almost took me out of college. With that threat, I promised to get rest, eat proper nutrition and no beer. I recovered without any complications. BTW, how do you know if you have Mono if you haven't seen a doctor?