I agree. Another issue with mental and emotional immaturity is that one is usually "stuck" at a certain developmental stage. They are not likely to "outgrow" this immaturity. So, while an older person and a younger person may be a good fit temporarily, the younger person is still developing and will soon outgrow the immature person. An 18 or 21 year old will quickly mature and realize that they are dating someone who, despite being older, is actually much more immature. They will realize that, if they are looking for something serious, the immature person will not make a financially stable partner, will not have the emotional maturity to be someone they can lean on in tough times, will not make a responsible parent, and will not be an "equal" partner down the road. The problem is not the age difference; the problem is the lack of maturity. The same problem can happen when two people are the same age but there is a big difference with respect to their level of maturity (been there!).

If one is mentally and emotionally immature, it might be best to look for other partners (regardless of age) who are also immature and looking for something fun/casual. Immature people can still date-- they should probably just seek out partners who are at the same level of maturity. It won't work if one partner is footing the bill, doing all of the "giving" and care-taking, etc. Relationships work best when two people are at the same stage of life-- which isn't necessarily about age.