Steve Bannon, head of Brietbart News and considered the architect of Trump’s campaign, made the remark to a friend in November 2017. Bannon also described the office as already having taken a toll on Trump, saying he believed the President had “lost a step.”

Bannon left his chief strategist role at the White House in August.

Vanity Fair – “I have power”: Is Steve Bannon running for President?
Business Insider – Steve Bannon reportedly joked to a friend that Trump is ‘like an 11-year-old child’
The Hill – Bannon joked that Trump is like ‘an 11-year-old child’: report