I don't have a great answer for this...I did Dopey because I was already planning on doing the 10th Goofy, then I wanted the Minnie medal...so if I'm doing those 3 races, why not do the 5K, too? I think I'd been running for about 6-7 years.
IMO - If you can follow a marathon training plan, and are in shape to run a marathon, you can do Dopey.
Physically, I think I could have done it sooner in my running "career" than I did, but I wasn't mentally ready. I had to know I could do a standalone marathon before doing Goofy (when I did Goofy, Dopey wasn't an official "thing" yet, though most people - myself included - did the unofficial Dopey of 5K + Goofy), and I had to know I could do a half before doing a marathon, etc. Not everyone else is like that, some will just do Dopey as their first marathon/half, etc. The other thing to consider is the time you'll put in training, because that is really the hard part* - take a look at marathon/Dopey training plans and look at the mileage and time you'll be putting in for the weeks/months beforehand - can you stick to it? Will you have the time/desire/discipline to follow through on your training plan? Are you cool with most of your weekend at WDW being eaten up by races/naps/recovering?
But I think the fact that you're seriously considering it means you're already on your way to deciding to just go ahead and do it...
*okay so the other hard part is the 4 early wake-ups.