Recently I saw a Tumblr post encouraging adults to buy "childish" products they like, http://m-to-the-6th-power.tumblr.com/post/165306196219/dont-feel-ashamed-of-doing-childish-things
LEGO is an obvious example, and I was all too happy to expound upon it.
LEGO by design has a lot of capacity to be what you want it to be, so it's easy to approach in a more mature manner. It's physically and thematically durable - the bricks are still usable and you'll still want to use them.
As an adult, I have more money and more experience looking for deals, so I can buy a lot of LEGO I couldn't as a kid. Now I can get big sets; I finally have a large castle and X-wing like I wanted as a kid. (I bought current equivalents, though it's quite possible to get the exact sets you missed on the aftermarket). Stamp collecting is called the hobby of kids and kings because there are ways to approach it at any budget; same goes for LEGO. Speaking of collecting, many adults approach LEGO as such and take good care of what they play with.
There are a lot of sets geared towards adults, more complex builds with thousands of parts that cost hundreds of dollars. #10251
Brick Bank, 2380 pieces for MSRP $169.99 is one of my favorite sets, and it's hardly the biggest.
LEGO has more female figures and sets now. (I feel that's progress even though some of the stuff is stereotypical, but that's a whole other post) I'm a guy who likes that, and it might help LEGO's appeal with some women as well as some girls.
The associated cartoons really do seem too simplistic for adults, but not the sets themselves.
"When I was ten, I read fairy tales in secret and would have been ashamed if I had been found doing so. Now that I am fifty, I read them openly. When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up." - C.S. Lewis