People born weighing less than 1.5 kilograms are usually shorter and have fewer friends and fewer educational qualifications than their peers in their formative years, but tend to catch up in their 20s, according to New Zealand research.

A study of all New Zealand's very low birth weight (VLBW) people born in 1986 since their birth followed more than 200 people now aged 22 or 23, and found they were now scoring similarly to their peers on a range of measures of health and social functioning.

While VLBW people tended to be more socially isolated and achieve less academically, they were as happy with their quality of life as peers, research leader Professor Brian Darlow of the University of Otago said in a statement Tuesday.

"This study is good news because it shows that, with some exceptions, these young people are doing pretty well despite serious hurdles early in life," said Darlow.

Despite clear evidence in their early years that VLBW infants had higher rates of problems such as cognitive delay and emotional and behavioural issues, the study found no evidence to confirm that these problems necessarily persisted into adulthood.

The study found that while VLBW people as adults were on average 5.6 kg lighter and 4.2 cm shorter than their peers and half as likely to have a tertiary qualification, it also found no significant differences in high school completion, involvement in paid employment and after tax income and close family relationships.

The next step for researchers was understanding if there were health discrepancies, such as premature ageing, between VLBW people and their peers, he said.