Tired of Being the Baby of the Family...

So, I'm the youngest in my family, which means I've been called 'the baby' way too many times to count. My parents still treat me like a toddler despite being 18 and trying to prove myself as an adult. The worst part is when my older sister acts like my mom, lecturing me on every little thing... It's not that they don't love me; it's just frustrating because I feel stuck in this perpetual kid role. Am I the only one who feels like this? Is it normal to still be seen as 'the baby' even after high school graduation? Anyone have any advice or been through something similar...?