Updated February 5th: read my full review of TR: Underworld here

Sum up:
Tomb Raider: Underworld starts right where Tomb Raider: Legend left off, and involves Lara looking for an artifact called, "Thor's Hammer." Like previous Tomb Raider games, Lara will be climbing, jumping, swimming, and shooting her way through the game's exotic environments.

A new addition to this game is enhanced environmental interaction; Lara's footprints will now stay in the ground, and mud/dirt will transfer to her knee when she kneels (both will be washed away if it rains). The rain will make climbable surfaces slick, making Lara slip if she's not careful, and Lara will realistcally push things like foliage in her way, etc. The combat system has also been enhanched, and not only does Lara have a new set of moves, but she can now fire at two different targets with either gun at the same time, and her grappling hook can be used to pull down/out puzzle objects.

Recommended age:
The ESRB rated this game "T" for Teen, listing "Blood," "Mild Language," "Mild Suggestive Themes," and "Violence." The "Blood" consists of very light spurts when Lara shoots an enemy. The "Mild Language" means there's a couple uses of words d*mn and hell.The "Mild Suggestive Themes" obviously refers to Lara's exaggerated features, movements and dress--but suprisingly, there is almost no use of breast physics.

And the "Violence" consists of Lara shooting and punching/kicking her enemies, and there's plenty of explosion and moderately intense situations in cutscenes and gameplay. This game is pretty mild overall, and although Lara's overall attitude, dress and movements are quite suggestive, I'd still recommend this game for ages 13+.