Rathbones' analysis assumes 8% annual investment growth and full use of the £20,000 Isa allowance. The firm noted that the number of Isa millionaires has at least doubled every three years since 2016. Angela Smith, Senior Investment Director at Rathbones, said: "Our analysis of the data suggests the number of Isa millionaires is likely to have risen significantly over a relatively short period, even on modest assumptions. This means many more investors are now benefiting from tax-free growth, income and dividends on seven-figure portfolios." She added that patience, time, consistent use of allowances, and avoiding interruptions to compounding are key to building such portfolios.
Recent data from Moneybox shows that Stocks and Shares ISAs opened during the 2024/25 tax year increased by 27% compared with the previous year. Notably, 21% of new accounts were opened by under-25s, indicating a shift towards younger investors. Brian Byrnes, Head of Personal Finance at Moneybox, said: "What's particularly striking is the momentum we're seeing in our own data, with young investors leading a cultural shift towards investing and taking greater control of their financial futures." Vanguard UK also reported around 225,000 clients aged 30 or under on its Personal Investor platform. James Norton, Head of Retirement and Investments at Vanguard UK, emphasised the importance of starting early: "Time is an asset and the longer you can be in the market, the better. Younger investors generally have more time to get to their goal. As a result, they can afford to focus more on maximising returns than preserving wealth."
Our analysis of the data suggests the number of Isa millionaires is likely to have risen significantly over a relatively short period, even on modest assumptions. This means many more investors are now benefiting from tax-free growth, income and dividends on seven-figure portfolios. Our experience with clients shows that the not-so-secret ingredients behind building an Isa millionaire portfolio are patience, time, consistently using allowances, and avoiding interruptions to the power of compounding.
In Sweden, a similar trend is evident with Investment Savings Accounts (ISK). According to TV4 Nyheterna, fewer than 3.8 million Swedes had an ISK in 2023, but by 2024 that number had risen to 4,218,556 — an increase of over 450,000 in two years. From January 2026, the first 300,000 kronor on an ISK will be tax-free, doubled from 150,000 kronor in 2025. Over 211,000 children under 16 now have their own ISK. According to TV4 Nyheterna, Christina Sahlberg, Savings Economist at Skandia, described the ISK as simple because it requires no declaration and offers an easy way to pay tax. She also noted that parents can open an ISK in their own name to save for children, maintaining control, or open one in the child's name to benefit from the per-person tax exemption, though the money then legally belongs to the child. Sahlberg emphasised that the most important thing is to teach children about money, not just save for them.
Rathbones also reported that global equities have delivered annualised returns of 17.4% since April 2023, though the exact current number of Isa millionaires as of the end of the 2024/25 tax year remains unknown. The breakdown of Isa millionaires by age group is also not specified in the available data.
What’s particularly striking is the momentum we’re seeing in our own data, with young investors leading a cultural shift towards investing and taking greater control of their financial futures.
Getting started early is really important. Time is an asset and the longer you can be in the market, the better. Younger investors generally have more time to get to their goal. As a result, they can afford to focus more on maximising returns than preserving wealth. In practical terms that often means holding more equities.
It's simple. You don't have to declare. It's an easy way to pay tax.
You can either have an ISK that you open in your own name and save for your children. Then you have full control over that money and can give it to your children at any time. Or you can open an ISK in the children's name. Then you get to take advantage of this tax exemption because it applies per person. But then it's very important to remember that then it's the children's money legally.
The most important thing is not to save for your children. It is to teach your children about money.