I bought my first yesterday (24) after contemplating it for a while: the Givenchy medium Antigona. I love it, am really happy with my decision, and am already planning my next (smaller sized) bag. It was money well spent.
The bag that got me interested in bags was a black and white SDJ by Saint Laurent. I almost bought it but I don't feel comfortable having a bag that doesn't close fully.
This is such an old thread but I had to add mine! My first real designer bag was a gorgeous, buttery red leather Marc Jacobs that I snagged for around $350 (originally $1200) I was probably about 20 years old and I felt like I was on top of the world! I still have the bag and use it occasionally and marvel at the beautiful, soft leather.