Until then, Lib Dems want youngsters to have a "truly play-based" education.
The party insists the change could be part of improving Scotland s education, tackling the attainment gap and giving youngsters the best start in life.
The issue is being raised at Holyrood after Scottish Liberal Democrat leader Willie Rennie used his party conference speech to call for every available teacher to have a guaranteed job, in a bid to cut class sizes and boost learning after the coronavirus pandemic saw school closures and disruption to learning.
Currently, youngsters start primary school when they are just four or five years old, with Liberal Democrat education spokeswoman Beatrice Wishart saying this practice dated back to the Victorians.
She insisted that raising the starting age for formal schooling was "an important part of our plans for the next Parliament to make Scottish education the best again".
Ms Wishart will use the Holyrood debate to set out the party's "commitment to making education truly play-based until the age of seven".
Ms Wishart insisted education will "still be mandatory" under their proposals, but would "focus on child development, social skills, outdoor learning, and physical and mental health".
She said: "Countries excelling in education and equity show that this approach better prepares children to shine in literacy and numeracy.
"They might start a bit later, but they quickly surge past us. By learning together through play, children develop the critical skills needed for better long-term development and outcomes. I want Scotland's children to get the same long-term benefits."
The Liberal Democrat continued: "The best way to close the attainment gap is not to open it in the first place.
"The Victorians didn't give us the best way to start school. Now we have the SNP conducting national testing of four and five-year-olds against the will of Parliament.
"Scottish Liberal Democrats will always be the party of education. It's time for a historic change to give our children the best start in life."
But a Scottish Government spokeswoman said there were no plans to increase the school starting age.