I would think it was odd for an 18 year old at uni not to speak to their parents for 3 months while at uni. Even 30 years ago so pre mobile phones/Skype I think it would be the norm to speak to them at least weekly.
I think a 21 year old should be more settled into adulthood. Eighteen has only just left school and may still be learning the ways of the world. We give our 19year old more practical and financial support than her siblings.