From a gameplay point of view, Portal is simply one of the best and deeply though out games I have ever played in my life. The puzzles are at times maddeningly difficult, but never impossible. Although Portal is now 5 years old, the graphics are still nice and run fluidly even on older machines.
Now for the problems. In the later levels, there are turrets that shoot at you, with moderately realistic blood splashing onto walls behind you. Turrets are also heart-breakingly adorable, and they make you love them even though they want to kill you. The main enemy of the game (GLaDOS) constantly taunts you with messages meaning to hurt you, and may upset younger players. Nevertheless, she is an excellent character and is undoubtedly one of the best video game characters of all time despite the fact that she is a megalomaniac who has wrested control over a science facility.
A final problem is the last level, where the player is running through the 'behind the scenes' of the facility. The environments are at times scary and disturbing, that unsettles even me.
Portal's fantastic puzzles create an excellent challenge for teens and above, but the mild violence and negative themes makes it unsuitable for pre teens.