who started the latest?

[Maxgaggi]

who was the world champion that startest boxing the latest?

[CarlosG815]

Antonio Tarver was 29 years old.

[Barn]

He wasn't a world champion but, Ron Lyle was 30 when he turned pro.

[Scott9945]

who was the world champion that startest boxing the latest?

Are you talking about boxing professionally, or just boxing, period?

[Kid McCoy]

Jess Willard made his pro debut at 29.

[Wild Blue Yonda]

Willard & Tarver, each twenty-nine, are the oldest I'm aware of to have won linear Championships.

Pretty interesting to note, too, that the other thing these two have in common is having won their titles from very respected, formidable Champions (even if both were over-the-hill), & each by heavy-handed knockout.

[hhascup]

Johnny Buff

Ring Record Book has him being born on June 12, 1888, Boxrec has November 26, 1891 and he started his Pro career on January 28, 1919.