(They also have monthly but not very serious contests on their page if you'd like to check)
Age limit: None
LEGO IDEAS invites you to submit new, original, and creative ideas in the form of a "Product Idea" that others vote on and we consider as potential new brick-based LEGO playsets.
Your product idea must focus on a single concept, similar to our own sets that feature a main model and a few smaller supporting models. For example:
" Don't attempt to pack an entire playtheme of models into a single product idea.
" We don't accept "mass customization" ideas such as custom mosaic or minifigure makers.
" Only reference one third-party property per product idea. For example, we wouldn't accept an idea containing both Porsche and Ferrari vehicles.
Members can "support" (vote for) their favorite ideas. Once an idea reaches 10,000 supporters, the LEGO Review Board reviews it and considers it for production using a process similar to the one used for our own LEGO products.
You can upload your submissions whenever you like - there is no deadline as they annually review the possible products thrice.

If your Product Idea reaches 10,000 supporters and is chosen to be made into an official LEGO product, you will receive:
" 1% of the total net sales of the product (this includes product ideas featuring original models based on third-party intellectual property such as a game, TV show, or movie). All payments shall be free and clear of any and all taxes, duties, levies, fees or other charges, except for withholding Danish taxes. For details, refer to the Terms of Service, (iii.) Taxes.
" 10 complimentary copies of your LEGO Ideas set.
" Credit and bio in set materials as the LEGO Ideas set creator.
If your entry that has reached 10,000 votes isn't approved in review, you will instead receive $500 in Lego products.