User blog comment:ShadowDragon135/Nice To Meet You... Again!/@comment-1986314-20120815010723/@comment-26113388-20120815145343

There are several theories why Tarble is 39, older than Goku, yet still looks and sounds so young:

Saiyan Heritage: Saiyans retain their youth so they can fight longer - common knowledge

Poor fighter: Maybe, he was born that way, it's often stated that Tarble is a weakling, maybe he was born with a small body, and that's why he can't fight

Genetics: Ever realise that Vegeta is much shorter than Goku? He's also shorter than several characters (barely taller than 1st form Frieza while on Namek) Maybe, that could play some part in it

Cut down?: This is gonna sound a little stupid (and somewhat gruesome) but maybe...just maybe, when the Saiyans realised he couldn't fight, maybe operations were made on him, taking out bones, muscles, etc. and given them to more deserving fighters who were in accidents? OK, so that one is just a guess but it could have happened