* Debut Years: Ajay Devgn made his debut in 1991 with "Phool Aur Kaante", while Akshay Kumar debuted in 1991 with "Saugandh". So, technically they debuted in the same year.
* Early Careers: Both actors had a few early roles in smaller films before they became big stars.
* Defining Success: Both actors found major success later in their careers with several blockbusters.
Therefore, it's more accurate to say that they both debuted in the same year, and both have had long and successful careers in Bollywood. It's not really about who came "first", but about the fact that they both made significant contributions to the industry.