I got my boys 1st hair cut on their 1st birthdays. They get a hair cut every 3 months now. My youngests hair grows so fast.
My DD had hers trimmed around 15 months and only because she looked like an elf. It pointed in the middle if her forehead and at her ears. Lol. My nephew has been getting his hair cut every 2-3 months since he was 6 months. His hair is so thick and curly.