With daily practice, I believe you can go a long way. The slower movement is 5th grade by AMEB, Australia - the difficulty with this movement is the voicing and balance between parts. The rest is at 8th grade - but just be very relaxed and precise with the left tremelos.
If you are mad enough to try it, try it! It won't hurt. Enthusiasm is a bad teacher but it can speed up learning enormously. Just don't expect too much in short time and be prepared to put the piece aside if it turns out to be too hard yet. Nine months ago I picked up piano playing again, after almost 28 years. Today I am better than I ever was. The learning curve doesn't depend on age, at least not yet (I am a bit older than you).
and if the piece i try isn't hard at all but i still can't play it without any mistakes? then what should i do? because i've been playing the italian concerto and the concerto in d minor(alessandro marcello) for over 3 months and i'm still not playing it well enough to get like at least 85% in the exams and i was told that i shouldn't be trying those pieces at 14. is that true? i personally don't think so though. what makes a piece unsuitable for a 14 yr old actually? complicated hand movements like the ones in the last movement of beethoven's 26th sonata?
I think that if you can read the music well enough and understand the principles of interpretation and practice, you are well able to play this Sonata. It will help if your hand is a good size, which it probably is as a man. Start out carefully and slowly. Practice moving your fingers well, aiming for the notes and working with each phrase for all its qualities. It will take some time and committment of regular practice to get your fingers 'fit' again, but there are many advantages to being a mature learner.With daily practice, I believe you can go a long way.
To be honest, if it took several hours to learn 7 bars of the Op. 13 sonata, you're wasting your time.Daniel