How old is too old?

If the 18 year old is looking to just score for a discreet encounter, then I would guess around 40 or so. For anything more serious and longterm, up to 26 would be the cutoff.

Probably no more than 25... just because it starts getting a little too weird and there's too much difference to keep the relationship going towards what you both want out of it.

38 years old I think is to old for an eighteen year old to date.

Here's a good rule of thumb. Subtract 7 from your age and then double that number. Any higher number is probably too old for you. So, in your case, 18-7=11, double that, 22. There's a little give, maybe up to 25. But more than that is probably a bad idea. Age gaps matter less as you get older though.