Generally, it is not illegal for these two to date. Of course, any sexual activity is considered statutory rape, but if there is no sexual aspect to their relationship, then they are probably fine. However, this is a fine line to walk for the 21 year old.

Is it okay for a 16 and 20 year old to date?

Its generally legal for a 16 year old and a 20 year old to have sex in OK.

Can I date someone 6 years older than me?

If someone older than the age of consent has sex with someone younger than the age of consent, its legally considered rape. If its illegal, you may feel like you need to keep your relationship with this person a secret.