You can’t always believe everything a big box pet store employee tells you. They are often uneducated about birds, and usually aren’t bird owners themselves.
The age has nothing to do with it. Regardless of age, any newcomer to the cage will be met with at the minimum a little pushing around until pecking order is clear. It’s best to rearrange the cage; major branches, eating areas, toys, etc. so that territoriality may be reduced.
The main thing to be careful of with mixed genders, is to keep the birds from breeding. If your dad would like to know why this should be prevented, here are some links explaining. Please read if you haven’t already.
Best wishes with your budgies