Deontay Wilder first stepped into a boxing ring to begin training as an amateur in 2005. He won his WBC belt in 2015. 10 years.
AJ first stepped into a boxing ring to begin training as an amateur in 2007. He won his IBF belt in 2016. 9 years.
Why is the number of fights important?
I don't think it's about the number of fights until they won their first title, it's what they've done after winning their world titles.
After beating Stiverne for the WBC title, Wilder had a 9 round fight against Molina (pretty much Molina wobbled him), then fought Duhaupas, Szpilka, Arreola, Washington, Stiverne again and then he stepped up and fought Ortiz.
After beating Martin for the IBF title, Joshua fought Breazeale, destroyed Molina in a complete mismatch, then fought Klitschko for another world title, then was supposed to fight Pulev but ended up fighting Takam because Pulev got injured, then fought Parker for another world title, then fought Povetkin.
This is most likely down to their management so i don't really blame Wilder, maybe Wilder wanted to fight Martin, Klitschko & Parker but Joshua got those fights because hes a bigger pay day for those fighters. That being said, it's not going to stop me from saying Wilder has a joke of a resume of a 40-0 world champion.