Can a 16 year old date a 18 year old?

Generally, there are no laws about dating.

They can date. In many places 16 is the age of consent to sexual activity, though in some places it is older or younger. It is important that you are aware of local legislation.

In some societies someone in their late teens dating someone in their early teens would be frowned upon, in others it would be quite acceptable, even encouraged.

It is necessary to conform to the ethics of the society in which we live, lest we offend the 'moral police'.

"Mrs Grundy" is everywhere. Offending her sensibilities is always punished. There is no appeal and you are guilty even if proven innocent.