Cecily - As a young mum, and having worked with young mums, it always half and half. Some of them formula feed because that's what they've known within their families and friends. Others decide that actually, they want to breastfeed, and they are more stubborn than mums in their 30s when they put their minds to it 
To be honest, I think age has very little factor within it. I always used to. Now I think its more personality; ie, how easily impacted they are from other people. I was 17 when I fell pregnant and lived the first year of my DSs life with my family, where my parents had formula fed us both from the off and couldn't understand the fuss I was making. DS is 2.8yr and continues to feed.
I once helped a 14 year old girl relactate. She started off breastfeeding, then stopped because her boyfriend told her she looked like a cow. A few weeks later he disappeared off the scene, and she approached me, asking for help to get started again. She did, successfully, and when she returned to school after her maternity leave, she continued to express.
Anecdotes, admittedly, but two that make me smile. Statistically rates of breastfeeding amoung young mothers is on the increase, actually, something I'm involved with in the local area.
We need to get rid of this idea that breastfeeding is something white middle class women in their mid twenties to mid thirties do :) Its changing.