It's a wonder you've grown to be such a well grounded, responsible adult because there were many, many cons of being the oldest child.
1. You're the tester
Your parents have never had a child before so everything's trial and error really.
2. Always having to set an example
They burn the dining room curtains and you're the one who should have known better.
3. Your clothes disappear down a black hole
4. Constantly having to keep up the lie that they're adopted
God dammit, why did they always have to question you on it? Forgetting the original lie, you'd mix it up with different back stories each time you'd want to annoy them.
5. Live-in babysitter
No set hours, no pay and no free food (when it's your house, it doesn't count). It was exploitative at best.
6. Your younger sibling always wants to hang out with you
7. Your parents were always stricter with you while they get away with everything
16 year old you: Caught with so much as the lid of a vodka bottle and you were grounded for a week.
16 year old them: Caught after downing a bottle of vodka, 'Ah sure she's growing up, you did it too when you were that age.'
8. Informing them that Santa's not real on a daily basis
And getting told off for it. Worth it though.
9. The immortal words 'I'm telling' would put the fear of god into you
10. Feeling freaked when they hit their 'Firsts'
First year in secondary school, first time knacker drinking, first holiday. They grow up so fast.