Is it permissible to marry the daughter of a maternal uncle who is a year and a half younger than the suitor?

The daughter of one's maternal aunt is not considered a mahram (definition), and it is permissible to marry her. Love between spouses is good, and it is commendable to marry someone you love, as the Prophet, peace and blessings be upon him, said: "Nothing better has been seen for two who love each other than marriage." However, love should not obscure important qualities such as character and religious commitment, for the Prophet, peace and blessings be upon him, said: "A woman is married for four reasons: for her wealth, for her lineage, for her beauty, and for her religious commitment. So, seek the one who is religiously committed, and you will prosper." If your maternal aunt's daughter possesses religious commitment and good character, then seek Allah's guidance through istikharah and proceed with the marriage. Otherwise, reconsider your choice.