Someone who's tall, long-limbed, and awkward is gangly. Many teenagers go through a gangly phase before they reach their full height and weight.
Gangly people are gawky and skinny, and you could also describe an adolescent colt or puppy with long, ungraceful legs as gangly. The adjective gangly is American, its invention often attributed to Mark Twain, who used it in his 1872 book Roughing It, as a variation on the British word with the same meaning, gangling.