As mentioned before there isn't enough info here but the age difference isn't likely to be a factor either way. If your dad was charged with GBH with intent (rather than just GBH, or ABH) then it can be a serious matter but it really depends on the specific details of the incident.
If the victim does "drop the charges" it doesn't mean your dad will not be prosecuted, especially for such a serious type of assault. The decision whether or not to charge is down to the CPS ultimately, not the victim.