robins,
That's en excellent question that many of us sooner or latter has to face. I did not to long ago (again for the third time) and like sewwhat89 says the 243 Winch. is hard to beat. I ended up buying my 12 years old a 243 Winch. and my buddy's son starting shooting at age of 12 his 257 Roberts,(good job sewwhat89), I have still put some more thought into it, now I have changed my views... I think the 308 Winch. is the way to go. Simply because a youngster would have something to grow into or to grow with. With the vast choice of bullets for the 308 Winch. it can be loaded to do what the 243 Winch. does and more. As the youngter grows, the bullet weight grows as needed. Though many Elks have been shot with the 243 Winch., the 308 Winch. is definately a better caliber for Elk. He can grow to shoot heavier bullets like the 180 Grains Accubond which at MV=2640 can be shot out to 1000 yards before it goes subsonic and at 400 yards has enough punch (1577 ft/lb)to take a nice bull elk to the freezer. /ubbthreads/images/graemlins/smile.gif
Bottom line, the 308 Winch. will do everything the 243 Winch. will and more if needed.
Later on, after a lot of practice and he is confident to shoot at long range, taking a mule deer at 700 yards... you still have enough gun and antelope at even longer distances. Just my opinion. /ubbthreads/images/graemlins/smile.gif
Good Luck!