When age differences are too important it is obvious that the relationship will not last very long. It usually fulfills some psychological needs. Under these conditions, lifetime of such relationships is generally very low. For obvious reasons, a 50 years old man in a relationship with a young girl in her twenties cannot keep up for long. The difference in lifestyle or biological rhythm will do its destructive work along the years. Durable couples look alike and live with the same energy. The couple with a big age difference carries itself the seeds of its degeneration. It is anything but sustainable. It responds to an immediate need, very often a psychological need.
The time when we go to bed, time to eat, what we eat, interest, movies, etc. For those who have already lived in a relationship with a much younger woman, you've probably experienced this kind of constraints. At the beginning it's fun, in the long run it actually becomes a constraint. Do you want to live the same lifestyle that your daughter?
There have been too many scam marriages with residents of poorer nations and all government started a hunt against arranged marriages in Canada, France and elsewhere. In Canada, our immigration lawyer has already brought us several sponsorship applications refused for too large age differences in the couple. It would be a shame to waste your time, your money and start a sponsorship process with a marriage to be denied at the end because you neglected this aspect.
One can legitimately ask the question why a young woman of 20 years agrees to marry a 50 years old man. What are the motives? It can happen sometimes that she feels the psychological need to replace an absent father. Who could blame her? But this can in no case constitute sufficient grounds to found a couple. When the motivation is financial, what can we seriously expect of such a relationship?