I agree with ‘make a point’ that “Age couldve been an excuse for a bigger problem”
It is best to use a reason/excuse that is least hurtful/damaging when rejecting someone, whether for a date, employment, housing.
In this case the follow up info says the girl is older, so the mother saying the boy is too young is a way to stop the date that does not involve a lie.
I disagree with the first part of the comment “if you are so intersted , pick up the phone and ask the mother”
Unless you are the shadchan you have no business in contact with the parents.