People aren't saying you are young and naive because you want to breastfeed - most young mums don't want to breastfeed. (I am a young mum and dislike all the stereotyping, but it's a fact based on figures, not a generalisation) - people are saying you are young and naive for ASSUMING that everyone "should" breastfeed or at least try, or that there are no "good" reasons not to want to breastfeed.
I am a breastfeeding peer supporter, and when I was first looking at the issues, deciding how to feed my baby etc, I did think similar - breastfeeding is free, more convenient, and better for the baby so why doesn't everyone do it? Now, I know a lot more - and I know that breast vs bottle is a personal choice thing, and yes, breastfeeding is better from a health point of view, but I don't think that anyone should feel forced to do something they don't want to do. Bottlefeeding from birth is a valid choice, as is breastfeeding for six months and then stopping, or mixed feeding, or extended breastfeeding to, say, six years. All perfectly fine and good choices. My role as a peer supporter isn't to persuade women to breastfeed, it is to support them to continue breastfeeding for as long as they want to.
It's easy to think "This is what I am going to do, and it makes so much sense that everyone should do it!" but just because something is right for you doesn't mean it's right for everyone else. We are fortunate in this country to have clean water and access to health services and information that makes it safe to bottlefeed, which means that mums who can't or don't want to breastfeed thankfully have a perfectly good second option.
Don't judge someone for a choice that they make just because it's different to what you would do. They might have reasons that you don't know about that they don't wish to share.