In my personal opinion, Minecraft is a game for all ages. I don't mind playing with 8 year olds or even 5 year olds. Not all of them are ranters and annoying babies. I play with my little brother (age 5) and he loves Minecraft. We play together every day and even though he builds dirt houses, has no idea how to craft he still has fun. And really thats the point of any game including minecraft. To have fun and be entertained. To see happiness in my little brothers eyes is all that matters not the age, not your level of expertise, not even if you can't write anything into chat.

Although there has to be a physical limit. I don't think any 1, 2 or 3 year olds are going to be playing minecraft since they have an under developed brain and there motor skills don't function well enough to be able to press keys down and click on a mouse.

So really what I'm saying is; it doesn't matter how old you are. You can't set an age limit on fun and happiness.