"Today, in 2019, if the company was a person, it would be a young adult of 21 and it would be time to leave the roost," they said.
Page and Brin started the search giant in 1998 in Silicon Valley.
Both founders promised they plan to stay actively involved as board members and shareholders, and lauded Pichai for his leadership of the company.
The pair still hold more than 50 per cent voting shares of Alphabet. According to an Alphabet SEC filing in April, Page holds 42.9 per cent of the company's Class B shares and 26.1 per cent of its voting power. Brin holds 41.3 per cent of the Class B shares and 25.2 per cent of the voting power.
Google has nearly doubled its headcount since Pichai took over as chief executive, growing from a company of 59,000 employees to 114,000 now.
Google's stock increased less than 1 per cent in after-hours trading after the news was announced.
- AP