Being the eldest is not all green. The normal sibling rivalry will happen but what sticks the most is parenting.
You are the guinea pig of their parenting techniques. You are the product of trial and error. You will bring them through phases of your life and their reaction to them is not all approving.
Being 24, my circle of friends are in the phase of settling down. Some are just starting to plan and some are so deep into the planning process that before we knew it, it's time to buy the bridesmaid dresses. I, myself, am still trying to adjust to the fact that I am 'losing' my friends to tulle, flower arrangements, wedding favours, mason jars madness. I can only imagine my parents, coping with me being out of the house every weekend, helping a friend or having a social life outside of the family meet-ups.
To cope with this change, they might say things that are insensitive. They might say to never return home. They might say things that made no sense to you that you get caught in disbelief. They might just see your disbelief as a sign of rudeness. They will make you cry with their words but always remember. They themselves are unsure how to react to things. You are their first. You will always be their baby. So to have you go out with someone else and attend events with you , they don't know what to make of it.
Just remember this before you go storming out of the house, saying how they don't understand what you're going through; they don't. They don't know what the normal mid-twenties people have to go through. We just have to take a deep breath and exhale. As many times as possible. We are the guide for the rest. Their guide for the rest.