As a rule, Russian girls mature earlier. Usually, by the age 19-22 they are ready to get married and have children. You will hardly meet a Western guy that young to be ready to tie the knot. This is one of the reasons why Russian ladies prefer older men as their spouses.
When it comes to international marriages the age gap of 10-15 years between the partners is very common. As statistics shows these marriages in their majority are very harmonious and successful. It lies in our human nature that women like maturity, stability and intelligence and men fall for youth and beauty.
Russian women are very family-oriented, so they seek for a stable husband, who would be able to provide for his family and take good care of his wife and children and offer respect and love. It can be a stereotype, but it is considered that older men turn to be better fathers and husbands, so a big portion of Russian women chooses a bit older men for marriage.
Marriages with 25 years difference also exist and some of these marriages work. However, this marriage is at high risk when a man grows older and his wife is still young and full of energy. These marriages work when the partners share many common interests, but anyway, the two need to be ready to conflicts of two different generations. Disregards your great love and crazy passion, when the romance is less bright and the routine days come the partners in the couples with 25 years difference need other things to hold them together.
Together with this, one needs to remember that not every Russian woman would like to get married to an older man. There are couples where the husband is a bit younger then his wife or they are even in ages. However, this works the best when men and women are in their thirties, both are mature and perfectly know what they need and want from life in general and from each other in particular.