My Curly/Spanish Mustang was gelded at 3 months. And he is 15.1. His dam was only 13.2 and his sire was only 14.1. The vet was coming out to do 3 other geldings so since his stuff was there, he got cut too. LoL. Talk about being at the wrong place at the wrong time.
He has great manners but he's part mustang so he's got a mischievous naughty "I'm going to test you" streak in him too.
My Curly was gelded at a year and a half. He was my first horse, we purchased him while still intact and never had any issues with him. He was just as calm back then as he is now. Never showed any signs of acting studdish.
We purchased a paint at 2 years old that was just gelded the week before. He was still healing when we brought him home. He was also the calmest guy ever. As a 3 and 4 year old, we had 5-6 year olds riding him without any issues. He was absolutely wonderful.