I have a tip for screenwriters and film directors. If you're going to take a storyline from an acclaimed, financially successful film you have to find a uniqueness, a way to improve upon the idea or at least have the decency to access that film's strengths and build upon that for your own film.
"13 Going on 30" takes a storyline similar to "Big", but lacks that film's innocence, charm and humor. In its place, it leaves behind more sensualized behavior, attitude and a general nastiness.
Jennifer Garner, so wonderful in "Alias," undoubtedly has acting skill but little of it is evident here. Mostly, she smiles widely and acts out a script that forces her to act stupidly being a 13 year old in a 30 year old body. It's a tremendous waste of her talent. All the sincerity and innocence that Tom Hanks put into "Big" is completely absent here. (Other than, of course, her ongoing shock and dismay at seeing a boyfriend's "thingy" and being appalled by the guys who hit on her).
Mark Ruffalo, in the role of Matt, her teenage friend who comes back into her life, does a nice job but really has no life or energy. Plus, I have to be honest here...I was troubled by all the other things I've seen Ruffalo in...He's played so many "sexualized" characters that I just never completely bought into his role here.
A joy to watch here is Andy Serkis, yes...that guy who does "Gollum" in the LOTR films does a relatively normal, fun role here...and is a nice addition to the cast.
There are some neat lessons here, but they unfortunately get lost inside the wooden writing and character development. Garner was never really convincing as she transitioned from an upwardly mobile, snobbish young lady to a more human, real and loving person who actually cares about someone other than herself.
In short, this is a below average film...while far from a complete waste of time, if you're really into this theme I recommend "Big."