Old doggie

I've had my border collie/australian shepard, Squanto, for about 12-15 years now. He has gotten so skinny. He usually is an inside dog, and eats inside (when he wants to). I try and make his food more yummy, buying his favorite, etc...but he's not putting on weight. Sometimes he will throw up, but it's not everyday. He also has very erratic behavior!
Is this just because he's old?
:confused:
:(

squanto in his younger years, more healthy looking.

http://s2.photobucket.com/albums/y43...ildlife003.jpg

He needs to see the vet. It could be any number of things, and only a veterinarian will be able to tell for sure what the cause is.