"When a construction site opens there's immediately a convention of pensioners who gather around the sides and say, shaking their heads: 'That's not the way to do it'," Renzi told a meeting with business leaders and members of the public in Florence.
"Their attitude is to say: 'We couldn't manage it. Of course they won't be able to'."
Renzi's comments followed a Rome rally which drew hundreds of thousands of protesters from across Italy - up to one million according to organisers - amid increasing resistance to his plans to boost growth and tackle unemployment.
Saturday's protest, called by Italy's largest trade union, mainly focused on Article 18 - a law which protects those who are unfairly dismissed.
Renzi wants to make it easier for companies to hire and fire employees in the eurozone's third largest economy.
He says Article 18 - drawn up in 1970 - is hampering efforts to get the recession-hit economy back on track.
"Clinging on to a 1970 law in 2014 is like taking an iPhone and saying: 'Where do I put the telephone token in?', or trying to put a roll of film in a digital camera. The Italy of rolls of film is finished," he said.
The former mayor of Florence, 39, has made the "Jobs Act" a cornerstone of his leadership in a bid to stave off a third recession in six years.
The plans still have to go before the lower house of parliament.
Youth unemployment stands at a record 44.2 percent and Italy - which has forecast a debt-to-GDP ratio of 133.4 percent in 2015 - risks falling further into the economic mire if it cannot return to growth.