Modern laptops (especially within the mid-range of price) often use integrated graphics in favour of having a discrete card, and no doubt a high percentage of PC users (especially in the younger crowd) are going to be using a laptop to play the game. Integrated graphics are still being made, and are most certainly not all 3 years old.

Mid-range laptops usually focus on one part, they either have a stronger CPU and integrated graphics or a weaker CPU and a dedicated GPU. That said, don't make assumptions about what system people use. That's not professional.

Plus, do you seriously expect the younger crowd playing Minecraft to have the sort of money, resources, support, ability, interest, or etc to build a computer?

If you want to play games, you need a system capable of doing so. This argument of /u/ColoniseMars still stands. Especially because MinecraftPE exists, most middle-class homes have at least one system capable of playing either Minecraft or MinecraftPE. Don't have a system to play games on? Save money or find another hobby. It might not be fair, it might not be fun, but all hobbies cost money.

Do you expect these people to shell out at least a few hundred dollars just to play the next update for a game they already own?

The most recent stable patch runs just as fine, if not better, as previous patches. The current snapshot is a poor example because as a WIP, there are no optimizations in the code. Keep that in mind.

Not to forget that the game does not prevent you from playing if you can't support an update. You can just use an older update using the launcher as if nothing happened. Preventing a game to grow just because a percentage of people don't have a system strong enough is a terrible reason for stagnation.