Probably a lego technic remote control car I made when I was about 13. A mix match of parts but it kinda worked.
But lets say in a more formal environment (with design drawings and models), would have been my first year of university so when I was 18. Made a pneumatics robotic grabber (uses air pressure to move). Didn’t manufacture one of the sealing parts very well, and it got launched into the blast shield around the machine, leaving a small crack in it.