The company, which announced a 10% increase in interim profits before tax, said since 2012 the average age of clients had dropped by seven years.
HL said that in 2012, 46% of clients were aged between 55 and 80 but that proportion is now only 34%.
The trend has continued during the Covid-19 outbreak with 47% of new clients in the 30-54 age bracket.
The company has invested heavily in digital technology including a new app which is proving popular. HL said during the half year its mobile app saw 57% growth in usage with clients checking their investments on average 10 times a week.
The firm said: “Younger people are taking a greater interest in investing for the future, recording an increased appetite for investment, and prioritising financial resilience and saving.
“Clients in this segment have money, are engaging with saving for the future and want help to put their money to work. By getting clients onto the platform earlier, we are able to support them for longer as they grow their wealth over time, and this enhances the lifetime value opportunity.”
In its interim figures, the firm said net new business was £3.2 billion for the six months ending on 31 December, up 40% on the previous year.
Assets under administration were up 16% to £120.6 billion, helped partly by a recovery in stock markets, and active client numbers rose 84,000 to 1.496m.
Pre-tax profit rose 10% to £188.4 million and the interim dividend was pushed up 6% to 11.9p per share.
Chris Hill, chief executive, said: “As our client numbers continue to grow, we are finding that younger people are taking a greater interest in investing for the future, with the average age of our clients continuing to fall.”
The firm said overall it had seen volatility but strong growth against the backdrop of “difficult” external conditions. Covid-19, the US election and Brexit all created challenges, the Bristol-based firm said.
It added that investor confidence had “fluctuated” over the period due to uncertainty in markets, falling over the summer but picking up into Q2 and through to December.
Investors were active in trading stocks with volumes up 123% on the previous year.
Mr Hill said the company remained optimistic about future prospects.
He said: “We remain excited by the structural growth opportunity in the UK savings and investments market and confident in our ability to deliver sustainable growth through the cycle. The Covid-19 pandemic has underpinned the importance of financial resilience and we are well placed to support clients with their saving and investment needs across their lifetimes.”