Ive dated 2 of my close friends, one was slighty younger than me, and his immaturity started to become more apparent. He ended up 2 timing me with his ex, and i could never forgive him for it. Ive not seen him or spoke to him since then, but then again i probabily would of lost touch with him, as soon as i started university anyway.
My second friend is a few years older than me. Ive never been so happy, i think we become so close friends because there was chemstry there. Its been almost 4 months and i have no regrets of risking a freindship for this.
Ive never had a close friendship with any of my friends that its like their a sibling type relationship. So i cant help you there.