Credit: Jayme Oak/

They grow up so fast!

Kourtney Kardashian revealed on her blog that her 10-month-old son, Mason, has reached a huge milestone. "Mason is walking!!!!!" Kardashian, 31, wrote on Thursday.

Mason, whose father is Scott Disick, has been enjoying the city life lately as Kourtney and her sister Kim Kardashian film Kourtney and Kim Take New York in the Big Apple.

While mom Kourtney shoots her E! series and opens up an East Coast Dash boutique, Mason stays just as busy. "I took Mason to music class," she wrote. "Because Halloween is coming up this week, all the kids dressed up, which was so adorable." Mason dressed up in a Pottery Barn spider costume.

And it sounds like he isn't complaining. "Mason is loving music class... It's so nice to see him interacting with different children."