When you go to Walmart or Target, you'll notice that LEGO sets are always located within the toy section. Toys for kids that is... To be fair, the main consumer of LEGO sets are kids. There are also some "kids" who are much older. These "kids" either never stopped playing with LEGO sets or maybe they took a break and came back to the brand. Most people would say that these adults are "nerds" or maybe "geeks." A whole community has developed and we call ourselves AFOLs or Adult Fans of Lego. There are conventions, web sites, groups to join, displays to go to, and all sorts of activities to be a part of the community.
Many AFOLs for LUGs or Lego User Groups. These groups of people meet to discuss their hobby, but also often set up their builds for the local community to see. My first foray into this area was a search on YouTube for "LEGO train displays." While some videos are made by kids crashing their trains, there are many videos of details displays of model LEGO trains transiting through LEGO landscapes. The detail and creativity is truly astounding.
Searches around the internet will bring up all sorts of creations. People have build model ships, cars, buildings, boats, and scenes to the extreme detail using all LEGO parts. Many of these models or MOCs as they are called (MOC stands for my own creation) consist of thousands of pieces and took months to build. Want to see some?
World War II destroyer - 10 feet long, 9 months to build
Townhouse with its own Starbucks
A huge castle
Pompeii (pre-volcano)
So before you make fun of that old guy/girl for still playing with kid toys, ask to see some of their work first. It may impress you and change your mind.