I always had the impression that hobbits lived about 100 years (how old was the Old Took? I can't remember if Bilbo was supposed to have exceeded the Old Tooks years, and I remember Pippin made some remark to Bilbo at the end that if Bilbo could outdo his great granddaddy, he might as well try to outdo Bullroarer) so fifty years being the mid way mark would easily put Frodo in the prime of life, equivalent of 30s for us, which is the prime of life for us. And coming of age at 33 would be like graduating at 18 from high school. If Frodo inherited the Ring at 33/18, then I would rather expect him to look rather young.
It never did look right to me to have Frodo look like some worn-out fat guy who spent most of his life drinking and smoking. Lij was perfect for the part.
Monaghan looked nothing like I had expected of Merry though. (ooops...there I go again!)