Allow me to give a brief history lesson about the Koopalings.
The Koopaling's debut was in Super Mario Bros. 3. At that time they were said to be Bowser's seven children. The Koopaling's would each invade their own individual worlds. By invading several kingdoms, they'd turning the kings into strange creatures using the king's wands, which they stole. During that time, Ludwig was the leader of the bunch, and Bowser's favorite Koopaling (thus making Ludwig the Bowser Jr. figure at the time) as mentioned in the Super Mario Bros. 3 handbook and Super Mario wiki.
Super Mario Sunshine came out in 2001 (I think), which was the debut of Bowser's "only" son, Bowser Jr. Because of this, Miyamoto decided to change the Koopaling's entirely. Instead, they are the leaders of the Koopa Troop under Lord Bowser, prior to them being his children. Now the Koopaling's stories and proposes were changed. As of now, Larry Koopa is the youngest of them and the leader of the Koopalings.