Kerry's attack came in response to Trump's tweet accusing the former secretary of state of holding "illegal meetings" with the Iranian government that were "to the detriment of the American people".
"He told them to wait out the Trump Administration! Was he registered under the Foreign Agents Registration Act? BAD!" Trump tweeted.
Reacting to the accusation on a late-night talk show on Friday to promote his new memoir 'Every Day is Extra', Kerry said, "He's the first President who spends more time reading his Twitter likes than his briefing books or the Constitution of the United States".
"Unfortunately, we have a president, literally, for whom 'the truth, the whole truth and nothing but the truth' is three different things. And you don't even know what they are," Kerry was quoted as saying by 'The Washington Post'.
"He really is the rare combination of an 8-year-old boy he’s got the maturity of an 8-year-old boy with the insecurity of a teenage girl. It's just who he is," he said.
Kerry's remarks received a mixed response online. While some lauded the comparison as "absolutely priceless" and a "mic drop" moment, many expressed disappointment for his "condescending" and "inaccurate" generalisation of young women.
Matt Summers, a spokesman for Kerry, later defended Kerry's Iran meetings.
"Secretary Kerry stays in touch with his former counterparts around the world just like every previous secretary of state," he said.
Kerry along with other top US officials played a key role in negotiating the 2015 agreement between Iran and several world powers that lifted sanctions against Tehran in exchange for restrictions on its nuclear programme.