The average Stocks and Shares ISA for 45- to 54-year-olds is said to be worth £25,362.

But how much could this be worth over the next decade? Well, to calculate this we’d have to make some assumptions as follows:

• Starting investment: £25,362
• Average annual return: 8%
• Investment period: 10 years
• No additional contributions
• Dividends reinvested
• No taxes due within the ISA wrapper

Please note that tax treatment depends on the individual circumstances of each client and may be subject to change in future. The content in this article is provided for information purposes only. It is not intended to be, neither does it constitute, any form of tax advice. Readers are responsible for carrying out their own due diligence and for obtaining professional advice before making any investment decisions.

Using compound growth, I calculate the portfolio could grow to around £54,754. That’s more than double the original amount.

Of course, future returns are never guaranteed. Markets move up and down, particularly in the short term. That said, history shows that patient investors who remain invested in quality companies should benefit from a rising long-term stock market.

But which quality companies could help drive these returns?

Could National Grid still be an attractive investment?

One company many income-focused investors continue to watch is National Grid (LSE:NG.). The business operates critical electricity and gas infrastructure across the UK. Because these assets are essential to everyday life, revenues tend to be more predictable than those of cyclical businesses.

National Grid has long been popular among dividend investors thanks to its history of shareholder payouts and defensive characteristics. It currently offers a 4.1% dividend yield with 2.4% dividend growth forecasted.

Looking ahead, the ongoing transition towards electrification, renewable energy projects, and grid modernisation could also create opportunities for future growth.

Bear in mind that it operates in a heavily regulated industry where rules are subject to change. Along with higher interest rates, this could impact future profits and shareholder returns.

In my opinion, its biggest attraction is its combination of steady cash flows and reliable dividend income. While it’s unlikely to deliver explosive growth like some AI-related technology stocks, investors seeking reliability might find its business model appealing.

Is now the time to think bigger about a Stocks and Shares ISA?

An ISA offers tax-efficient growth, flexibility, and the opportunity to build wealth without capital gains or dividend tax liabilities. Even relatively modest sums can potentially compound into surprisingly large amounts over a decade or more.

The numbers get even more appealing if fresh cash is contributed during working years. For instance, let’s assume the investor above makes one change to the listed assumptions. This time, they add and invest an additional £3,000 a year.

After 10 years, I calculate the investor could have a total pot worth around £98,213.

I think investors could be underestimating how powerful the effects of modest investments combined with the magic of compounding can be. Patience and regular contributions could significantly strengthen retirement finances.

In addition to the likes of National Grid, owning a selection of quality growth shares could significantly boost annual portfolio returns. For instance, Alphabet (NASDAQ:GOOGL) and Apple (NASDAQ:AAPL) have respectively returned 26% and 29% annually over the past decade.

All things considered, I think the real question isn’t whether a Stocks and Shares ISA can help build wealth. It’s whether investors are making the most of the years before retirement. And if not, what opportunities could they be overlooking today?

Should you invest £5,000 in National Grid Plc right now?

When investing expert Mark Rogers and his team have a stock tip, it can pay to listen. After all, the flagship Twelfth Magpie Share Advisor newsletter he has run for nearly a decade has provided thousands of paying members with top stock recommendations from the UK and US markets.

And right now, Mark thinks there are 6 standout stocks that investors should consider buying. Want to see if National Grid Plc made the list?

Harshil Patel owns shares in Apple and Alphabet.