Justin Bieber's Dream Mansion -- Underground 21+ Entertainment

2:10 PM PDT -- Sources tell us ... Bieber is extremely close to sealing the deal on the house ... but he does have one minor reservation -- he's frustrated that everyone knows about the home.

Still, we're told Justin and the seller are "very close" to going into escrow ... they just need to work out a few minor points.

18-year-old Justin Bieber won't be able to enjoy his L.A. dream mansion to the fullest -- at least for the next three years -- because one of its coolest amenities ... is strictly for the 21+ crowd.

As we previously reported, Bieber has put an offer on a 9,000 sq. ft. Calabasas estate way outside Hollywood -- and among its awesome features ... a fabulous wine cellar.

Bieber won't be able to avail himself of the cellar's services -- unless he plans to store Pepsi down there -- but he WILL be able to enjoy the 6-bdrm mansion's GIANT outdoor pool, spa, sauna, separate guest house, and private movie theater.

As for the real selling point -- the Calabasas community where the house is located is a veritable anti-paparazzi fortress ... with several highly-guarded security gates. The community is also home to Travis Barker, NFL star Keyshawn Johnson, and Master P's son Romeo.