To be blunt, no, Twinkies do not last forever, or decades, or years. The shelf life of a Twinkie is now 45 days, up from a previous shelf life of 26 days. Forty five days is a long time, but no where near close to years or decades. The misconception comes from another misconception that Twinkies are made from chemicals and are therefore not real food. Don't even get me started on 'chemicals'. All food products are made of chemicals. Water is a chemical! So to say Twinkies are made of chemicals and this causes them to stay fresh forever. Of course they are made of chemicals! Twinkies are no different than bananas or apples in that regard.
This misconception presents a great opportunity to setup your own science experiment with your kids. Buy a box of Twinkies and investigate them over several months, or even years if you have the patience. How does the Twinkie change? Does its smell change? Texture? Look? Ask you kids to note the changes. Twinkie science!