First, I think anyone going to graduate school needs to evaluate what their professional goals are first and foremost. Graduate school is a big commitment. Make sure you’re going to grad school with specific career goals in mind that are only achievable with the degree you’re seeking. Don’t just go to grad school because you don’t know what you want to do with your life. Sounds like you already have a goal in mind, but I just wanted to
Have you taken steps to manage your ADHD since you were in undergrad? (i.e. counseling, medication, etc). If you haven’t already tried these measures, it might be good to look into. I know someone who medication made a world of difference for! Also, I’m not sure if MFA programs require/recommend taking the GRE, but that might be a good way to gauge how your focus has come since undergrad, as typically most people study for the GRE. The person I mentioned earlier started on medication for ADHD after grad school and found studying for the GMAT much more feasible than trying to study in undergrad.
I’d hardly consider 25 “growing age”! I’m in graduate school now (biomedical sciences Ph.D. program). I started at 25 and was younger than many, many people in my program. Age is just a number 
Good luck!