Why does everyone say 'not to be rude but your 17' when you comment your opinion?

Li
I don't see why because I'm 17 I can't be right. It's like I'm not allowed to be right or have a opinion here! I want to say that age doesn't define knowledge or maturity! Just because I'm 17 doesn't mean I don't know the things a 25 year do and just because they're 25 doesn't mean they know the things I do! We are all educated in different ways and some of us are better on certain topics. 
I commented on a post about a women saying how anyone below the age is 18 shouldn't shop at VS and someone called it being a responsible parent. I'm not saying any names because I just want to say how I feel! 
I perfectly said just because your 13+ doesn't mean you shouldn't shop there. It's not about what BS sells its about what bras fit you best because honestly some teenagers have big breasts for their age. It's also about quality and I'm sure you would rather spend 40$ on a good bra then one that'll break in a few washes. Just because I'm 17 doesn't mean I don't have common sense.