>>4 7th graders are generally 12-13 with the few exceptions that are 11 or 14(skipping a grade or being held back a year). Sophomores are 10th graders, usually 15-16 with the few exceptions that are 14 and 17(again, skipping a grade or being held back). So presuming that neither are an exception, there is easily a 3 year(give or take a few months) age difference. For adults, this isnt uncommon. For teenagers and children, the maturity levels are most likely far apart.
>>1 For your situation, this gap is most likely a problem for any serious relationship. Being friends is fine, but having any serious relationship may be too much for the 7th grader. With the information you gave, I would advise keeping your relationship at a friend level. Next time, please post something like this on the dating thread.