Britney Spears has largely kept her two sons out of the spotlight, but on Monday she revealed just how much Sean and Jayden have grown. The two boys -- now 15 and 14, respectively -- are now taller than their mom.

The "Toxic" singer, 39, shared an adorable family photo of her sandwiched between her two sons, both of whom she shares with ex-husband Kevin Federline.

"It’s so crazy how time flies .... My boys are so big now," Spears captioned the post. "It’s very hard for any mama, especially a mama with boys seeing them grow up so fast."

"Talk about enough to make you go to your knees... GEEZ," she joked before expressing how proud she was to be their mom. "I’m extremely lucky because my two babies are such gentleman and so kind that I must have done something right."

As for why Spears is only sharing photos of her sons now, she revealed that she had been waiting for their permission.

"I haven’t posted pictures of them for some time cause they’re at the age where they want to express their own identities and I totally get it," the singer explained, nodding to her experiences growing up in the limelight.

Still, that didn't stop Spears from being a proud momma.

"I went out of my way to make this cool edit [of this photo] and guess what .... They’re finally letting me post it," the singer said. "Now I don’t feel left out anymore."

While she is stoked that she can finally start sharing photos of her boys, Spears said she wants to be a "cool mom" and therefore wouldn't be making too big of a deal out of it: "I’ll just read a book instead."