Let's split this into two categories: Consoles and PC (desktop and laptop)
Consoles - Once you buy the console you can play any game that has been designed for that console. The hardware basically stays the same until a new generation is released. A PS4 game released in 2020 will play on a PS4 you buy today (assuming it does not break) because the hardware for it will be the same compared to a PS4 bought in 2020.
PC - New hardware is always coming out. That means new games are always pushing the performance envelop. In many cases a newly released game runs poorly on current hardware and you need to wait for future hardware to max out graphic settings and also get good performance. Best example would be Crysis (the 1st game), when it came it even very high end PCs ($2,000+) could not run the game properly. Playing a 2020 game on a PC bought in 2014 (with no upgrades between those years) is likely to still be possible if it was a pretty powerful 2014 PC, but graphic settings would need to be turned down.
PS4, PS3, or XboxOne - I don't know. I do not play console games, Look at the available games and decide. Are there any exclusive games you want to play? You like playing Halo? Well, then you have no choice... Xbox One.
PC desktop or Laptop - Desktops are basically big and bulky, but they are relatively inexpensive unless you are really looking for a lot of performance, then the price tag can go up. Desktops are generally upgradeable. Buy a desktop today with a nvidia GTX 750 Ti and you can most likely upgrade to a GTX 970 Ti (not a real graphics card... yet) a few years later when it gets released.
Laptops are all the rage, They are small and portable. However, they are basically not upgradeable besides the hard drive and RAM. Expensive gaming laptops ($1,400+) give you the option to upgrade the graphic card, but those cards can be quite expensive ($400+). Additionally, laptop graphic cards a weak in comparison to desktops PCs. To get the same performance of a $600 desktop, you may have to spend $1,200 or more in a laptop.