This week Twitter entered the S&P 500 index on Wall Street - replacing 20th century, old economy giant Monsanto.

Another tech stock Netflix will replace Monsanto - which has been taken over by Germany's Bayer - on the S&P 100.

It was just the latest example of a growing trend that is changing the make-up of markets around the world and increasingly the way investors need to look at stocks, says PIE Funds chief executive Mike Taylor.

"In 1965, tenure of S&P 500 companies was 33 years," he says. "In 1990 it was 20 years. It's forecast to drop to around 14 years in the near future and 50 per cent of the companies in the S&P 500 will be replaced in the next 10 years."

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The new entrants will almost inevitably be dominated by high tech "software as a service" (Saas) companies - or what are sometimes known as cloud computing providers.

While the internet has been with us for decades pace of change since likes Netflix, Spotify, Air BnB and Uber arrived has accelerated rapidly.

Perhaps more importantly the template for disrupting an industry is now more widely understood by investors pushing billions of dollars of capital towards tech start-ups.

Traditionally you valued a company on its discounted cashflow on the assumption it would make profits and profits would grow, Taylor said.

Now many of the biggest and most successful stocks are companies - like New Zealand's Xero - that don't make profits and aren't even aiming to yet.

"They are a challenge because if the growth stops there is no profitability or dividends to fall back on. You rely on the company continuing to grow at a faster and faster rate and continuing to acquire customers."

But the big growth stocks couldn't be ignored by serious investors because the performance had been so strong, he said.

"We are in that golden age of disruption where these old world industries are getting disrupted," he said.

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There were risks around the longevity of the new tech players and whether they could make it to profitability but typically once an industry was disrupted once it was less likely to be disrupted again - to the same extent, he said.