The September issue of Nikkei entertainment magazine published a list of the top 50 manga creators by sales since January of 2010, along with other very interesting statistics:

Eichiro Oda, age 36, is in no doubt the most successful mangaka in the world, at the top of the list with 54 million sales of his trademark manga One Piece.

He is followed by Masashi Kishimoto, also age 36, his classic series Naruto has sold 11 million copies (in 2010 alone).

In the third place is Kimi ni Todoke's author Karuho Shiina, age 35, with 8 million copies sold.

The rest of the list can be seen here,

The average age of a succesful mangaka is around 30-37 years old if you look at the first 15 spots, but I would group it in the 30-40 years old group, which means that it takes quite a few years to train yourself to be a succesful artist and then get ahead with a series that will be picked up by a publishing company at around ages 25-28.

So, for a mangaka, absolute success comes later in life.

In Bakuman, the fact and Mashiro and Shuujin are almost serialized at 17 years old is pure fiction, which doesn't necessarily mean that there can't be manga authors with great talent at that age, but someone like Niizuma Eiji is a stretch of the imagination.

Beelzebub's author, Ryuhei Tamura, came in spot 21 with 2.7 million copies sold.

Yana Toboso, author of Black Butler, came in the 26th, with 2.3 million copies sold.

Author Kazue Kato of the highly acclaimed series Ao no Exorcist, came at spot 31 with 2.2 million copies sold.

And Akiko Higashimura, author of Kuragehime, came at 43th with 1.6 million copies.

Source: ANN