What does Islam say about parents not blessing a son's marriage just because the lady he is marrying is 3 years older than him? Bear in mind the son is 36 years old.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If this woman is pious, follows the rulings of the Sharee’ah, and has a good moral character, then convince your parent for this marriage. Remind them of the saying of the Prophet : “A man marries a woman for four reasons: for her wealth, for her lineage, for her beauty as for her religion. Select the religious one; may you be blessed.” [Al-Bukhari and Muslim] Also, tell them that marrying a woman you do not love is not good either for you or her. You may seek help from those relatives who can convince your parents to accept the bride you choose. Inform them that the Prophet had married Khadeejah who was fifteen years older than him. In addition, keep supplicating Allah to guide you to do only that which is good for you. Try your best to have your parents agree to this marriage; if they do not change their mind, then we advise you to obey them. They have much experience and always think for the benefit of their children. If you marry this woman without their consent, this might sever your relations with them, and then you may not fulfill their rights and consequently you may become undutiful towards them. The anger of the parents might result in the Anger of Allah as you know the right of parents upon their children is above all other rights. This right is so great that Allah linked their right to His. Allah Says (what means): {And your Lord has decreed that you worship none but Him. And that you be dutiful to your parents.} [Quran 17:23] Allah also Commands Muslims to be thankful to Him and immediately after it, He Ordered them to show gratitude to their parents. Allah Says (what means): {…give thanks to Me and to your parents, unto Me is the final destination.} [Quran 31:14] Therefore, ignoring their rights is to destroy oneself.

Allah knows best.