Who says it is not acceptable? Did prophet muhammad(peace and blessings be upon him) not marry khadaji(as)? Prophet muhammad(pbuh) was approximately 25 years old at that time and khadaji(as) was 50. Does that not tell you that prophet muhammad(pbuh) married an older women? Then ,why should we think it is unacceptable to do the same as our beloved prophet muhammad(peace and blessings be upon him) for that reason,i do not think it is not acceptable for a younger man to marry an older women. I don't think age is neccesary,in a relationship you have to attain a strong,communication bond.
For a proper marriage(regardless of the age of the two spouses) you must attain a good sense of communication,proper understanding,love,trust and faith. We need to have these qualities rather than care about the age. In conclusion,i do not think that it is unacceptable for a younger man to marry an older women,and my post explains why.