They are demanding, love a bargain, and are "promiscuous" when it comes to brands. Now this 20-somethings group of Generation Y consumers are set to leapfrog Gen X-ers and overtake Baby Boomers as the nation's biggest spenders.

And companies are paying attention as they target their products to the shopping personas of each generation.

"In the next 10 years we'll see a down-ageing of the key demographics brands will have to sell to, like switching from 65-year-olds to 35-year-olds in a short period of time," said British trend forecaster Chris Sanderson.

"They're totally different and many brands are at risk of falling behind."

According to his research, Gen Y consumers, in their 20s and early 30s, do not feel a sense of loyalty to brands and are happy to mix luxe pieces with bargain bin finds. Digitally fluent, they want and expect brands to engage with them online.

Sportswear giant Nike was a solid example of a brand that had secured Gen Y's attention and dollars by harnessing the power of social media, well-designed apps and community.

"They understood the power of check-in and location software so users could find other people say playing basketball in their town," he said. "Using their app, you can track where and how you run. Other brands are still playing catch-up."

Among marketers, Baby Boomers, now mostly in their 60s, are being called "flat agers" because they do not want to be defined by their age. They are travel happy, meaningful spenders.

Based on that insight, Luxury brand Yves Saint Laurent played the concept of "agelessness" right when it advertised its pricey, saffron-infused Or Rouge face cream as a "renaissance treatment" rather than another anti-ageing product.

"They don't want be told they need to buy anti-ageing products. They want to be told they are beautiful just as they are," Sanderson, co-founder of The Future Laboratory, said.

Next are the "multi-tasking" Generation Jones-ers, now in their 50s, who are community-focused, cautious spenders. They prefer high-quality, sustainable products from brands with a strong back story.

"In Australia, Country Road is the obvious example, as its ads represent an Australian way of life. It's not designer and not overpriced, but there's quality," he said.

Mr Sanderson considers Generation Jones as a standalone category. But some experts, such as Tony Dimasi, managing director of retail at research firm MacroPlan Dimasi, groups them with the Boomers.

According to Mr Dimasi's latest research, Gen Y will spend more than Boomers by 2021, increasing their share of total retail spending to 30 per cent. Between now and 2021, Boomers' share of retail spending will drop from 31 per cent to 26 per cent.

"Affluence is largely driven by income but not entirely so. One of the reasons why Gen Y are able to spend significant amounts is because their Baby Boomer parents provide the means for them," he said.

He said Generation X, those in their late 30s to late 40s, suffers from the "Prince Charles syndrome" because they are "sandwiched" between the Boomers and the ever-demanding Gen Y.

"Although we had strong economic conditions all the way through to the GFC, the Boomers called all the shots, and they're still hanging on. The readjustment post-GFC is geared more towards Gen Y, than Gen X," he said.

Mr Sanderson labelled Gen X consumers as "stressed out," having to raise children, look after ageing parents and pay the mortgage. Plus, with rising costs, they are frustrated they cannot splash their cash as they would like.

While Gen X are the most brand loyal, they are constantly seeking value-for-money products at no-hassle stores such as Target, Big W and Aldi.

But Dee Madigan, creative director of Campaign Edge who has worked on Coca-Cola, David Jones and Unilever campaigns, said Gen X were becoming less brand loyal because they did not want to grow up.

"We don't want to be like our parents stuck on buying the same brands, we want to be the cool ones still," she said. "In that sense, Gen X is acting more like Gen Y."

But if a brand attracts Gen X, it risks losing the interest of Gen Y. For example, when Zara opened its flagship Pitt St Mall store in 2011, the General Pants store opposite put up balloons and a sign that declared: "My mum shops at Zara".

"That's good Gen Y marketing. To make people say, 'that's an old person's brand," she said. "Also, brands tend to skip a generation before they're cool again, like Levis jeans. No one wants what their parents have got."

The Future Laboratory describes today's teenagers as the "visually led" Generation D, digital natives heavily influenced by their peers on well-developed social networks.

Even younger are the "phygitally active" Generation I who are so immersed in technology that they do not see it. They are also "interactive and intuitive".

On Saturday, the Moraitis family of Newtown went shopping along the ever-bustling King Street.

Casey Moraitis, 63 and recently retired, said when she sees a young model spruiking an age repair product, "It's like, get a grip. In some ways, it's offensive because I've earned these stripes."

Her son James Southan, a 37-year-old mental health nurse raising a young family in Newtown, said he has worn Dr Martens shoes his whole life, and was not planning to change anytime soon.

"When I'm shopping the focus is on buying affordable things of good quality," he said. "I think with my Docs, there's a childhood association that it's of quality."

His daughter Ruby, 5, was "very intuitive" when it came to technology. "Kids pick technology up so quickly."

Mr Southan's sister Elizabeth Moraitis, a 25-year-old nursing student, said she enjoyed mixing Australian designer brands such as Dion Lee with budget labels such as Supre.

"That's how I like to shop. I don't like to go full designer, because that's tacky. It's too much," she said.