People go out and even get married to people 30 years older than them and it's never regarded as such. In fact, most societies consider it commonplace or even proper for the guy to be a few years older than the lady.
I think it depends on how old you are. If you are 15 and he is 20 than I think that's dating too old. I think your maturity level is growing at rapids speed in your teens! However if one is 35 and dates someone who is 40 no big deal. I know it's hard to make sense of. Any respectable guy over 20 should date no younger than 18, in my opinion.
In most areas of te USA age does make a difference when dating especially for those still under the age of 18. Teachers can be legally prosecuted for dating a student. Man or woman. Teachers are not the only ones.