It is really quite unusual and very likely won't last. I think 20 is definitely a bit too immature, at a very different stage in life. The 20 year old needs a few more years before it would be semi-acceptable, maybe 25.
That said, at these ages they are indeed both adults and can do what they want in the end. Given this fact, you may as well try to assess whether it would be healthy for it to go ahead... The compatibility for a long term relationship is really small so I would say it is not a good idea. The 20 year old should find someone closer to her/his own age.