Mary-Kate OLDsen

Although she's just two weeks shy of her 21st birthday, Mary-Kate Olsen is looking rather aged these days. Being a multi-millionaire bag lady is a hard life!

Sources on the set of Showtime's "Weeds" tell TMZ that the newly-joined cast member is needing to spend a little extra time in the makeup chair -- to cover up her dark circles and other assorted blemishes! A well-placed source tells TMZ that several crew members were overheard commenting on how surprisingly old she looks in person. Where's a pair of oversized sunglasses when you need 'em?!

So why is MKO aging faster than Trader Joe's bread? One possible answer: we're told that when MK is not filming a scene, she spends all her time smoking Marlboro Reds and drinking her trademark Venti-sized coffees. The breakfast of child star champions!

Luckily for the celebretwin, our source says producers are ready to make any necessary cosmetic fixes in post-production. Editors are the new plastic surgeons!

A rep for "Weeds" could not be reached.