Hayden Panettiere celebrated her 18th birthday Tuesday in a big, important way ... by registering to vote at the Declare Yourself offices in Beverly Hills!  

Oh, and Hayden Panettiere also held a large birthday bash.

The actress and 75 friends and family partied at LG House in Malibu. On hand to help Hayden drink Diet Cokes were Stephen Colletti (Hayden's oh-so-lucky boyfriend of a year), BFF Rumer Willis and random family hanger-on Mario Lopez.

Also present were Heroes co-stars Adrian Pasdar (and wife Natalie Maines of the Dixie Chicks), Milo Ventimiglia, Greg Grunberg, Zachary Quinto, and Kristen Bell.

The food was Italian with pasta and Japanese with different types of sushi and rolls. The red velvet cake said (wait for it) "Happy Birthday, Hayden!" Fitting.

Having just turned 18, we suppose it's acceptable that Hayden Panettiere needs some blowing lessons. It took her five tries to get all those candles out!