There are many reasons for why the Prophet PBUH married Aisha, may Allah be pleased with her, when she was so young. I will mention a few things here:
• There was no force in marriage and she married out of her own free will.
• She was the only wife of he Prophet PBUH who never married before. All other wives were previously married
• The Prophet PBUH remained happily married to his first wife, Khadija, may Allah be pleased with her, for 25 years (until her death) and never felt the need to marry anyone else while being married to her. This was despite her being 15 years olds than him.
• The Prophet PBUH missed his first wife Khadija and gave gifts to her friends years after her death.
• Aisha, may Allah be pleased with her, felt jealous of the first wife of the Prophet PBUH and asked if Allah had not given him better -i.e.: referring to herself. This made the Prophet PBUH so angry that his face turned red and he swore by Allah that he had not been given better than Khadija, may Allah be pleased with her.
• People mature at different ages. Aisha, May Allah be pleased with her, was exceptionally clever and even after the death of the Prophet PBUH many senior companions consulted with her and asked her for advice. She is one if the greatest narrators of Hadith or sayings of the Prophet PBUH. Perhaps due to her young age, she was also very inquisitive and asked about things that others might not have even thought of asking.
• People have different marriage customs and until recently people married fairly early. Even today, the official age of marriage in many Western countries is 18 years but you will find many young girls who had children outside marriage as young as 13 years. Many of them also end up as single parents and receive government benefits such as in the UK.
• In medieval Europe, which was hundreds of years after the time of the Prophet PBUH, women would get engaged or betrothed very young such as with queen Mary I of England. Some women were engaged as young as 3 years of age. In some parts of the world, child marriages still exist such as in India and there was even a successful drama series about this called “Balika Vadhu”
• The Prophet PBUH is an example for us for how we can live our lives. A large part of life is getting married. As the Prophet PBUH had different lives throughout his life, these are examples for how we should live our married lives. This includes having an older /younger wife. For further details on this you can watch my YouTube series about the Wives of the Prophet PBUH. You can see this from here: