25 and 33 it's not "much younger" at all. I guess it's like third of the all couples in the world falls in range of 5 - 10 year gap. But I guess uncle is just looking for excuses to distance himself.
Divide by 2 and add 7 rule. There's the two year timeskip, so Yuichi is 35 now. (35/2) + 7 = 24.5, as in the youngest Yuichi should date is 24.5. This girl is already 25, so it checks out.