With arrows like IVF in their quiver, it's getting easier for young people to justify putting off their first pregnancy for longer and longer.

But when it comes to fertility, there's only so much science can be weaponised to suspend the consequences caused by ageing.

Eugenie Prior's study found an overwhelming number of students wanted to be parents. ( Supplied )

So when does time really start running out?

New research from the Royal Women's Hospital, Melbourne University and Victoria's IVF regulator suggests it's much sooner than many people think.

A survey of 1,215 students from the University of Melbourne found less than half could pick the age period when fertility in women declines significantly — for the record, it's 35 to 39 years old.

Fewer than one in five students could pick when men reach the twilight of their fertility years — between the ages of 45 and 49.

Fertility experts say the research points to a broader phenomenon of young people putting off having a baby because of "unrealistic expectations" of what they want to achieve before having children.

The paper's lead author, Eugenie Prior, says it's a worrying precedent.

Fertility in women declines significantly between the age of 35 and 39. ( www.sxc.hu )

Most of the university students she interviewed expressed an overwhelming desire to be parents one day.

"But there's so many things that they want to achieve before they have children," Dr Prior said.

"They want to be advanced in their profession, they want to be financially stable, they want to travel, they want to be in a stable relationships," she said.

But those ambitions exist in contradiction to the biological reality that many young people face.

"Fertility is one of those things that does have finite limits, even with all the medical technology that we have," Dr Prior said.

"We still know that fertility does decline and … maybe it's not realistic to achieve all of those things before you actually have children."

IVF no silver bullet

While the study's sample is small, it echoes trends being seen in the fertility industry.

"More and more patients are presenting later in life to plan a family and often to plan their first baby over the age of 35," said Raelia Lew, a fertility specialist and one of the paper's co-authors.

"In that group of patients, I do see a disproportionate burden of age-related fertility problems."

Dr Lew is one of only a few gynaecologists to hold the Certificate in Reproductive Endocrinology and Infertility, the highest level of postgraduate training in IVF and fertility offered to Australian and New Zealand doctors.

She said there was a dangerous misconception over the use of IVF, where embryos are created outside of the body and then placed in a woman's uterus.

Dr Raelia Lew, a reproductive endocrinologist, says IVF isn't a silver bullet to age-related infertility problems. ( Supplied )

"IVF is fantastic technology and there are literally millions of babies in the world that wouldn't be here if it wasn't for IVF," Dr Lew said.

But she said it wasn't a cure for age-related fertility problems.

"We're not fundamentally doing anything in IVF that reverses, treats or assists the process of egg deterioration that happens with advancing age," she said.

IVF is also expensive, costing up to $10,000 per cycle, although couples can claim back about half the cost through Medicare. Most require several cycles.

And it's no guarantee of success, according to Louise Johnson from the Victorian Assisted Reproductive Treatment Authority, which regulates IVF in the state.

"We all read the magazines and newspaper articles about miracle babies, but for many IVF treatment isn't successful and age is something that IVF treatment will not necessarily cure," Ms Johnson, also a study co-author, said.

"We need to educate young people about the limits of fertility."

So where does that leave potential parents?

"You've got to prioritise things that are meaningful to you," Dr Lew said.

"If it's really important to ...have children in their future, think about how that might happen and the context that it might happen," she said.

"It's something that can really save a lot of heartache later on."