By the age of three disadvantaged children show signs of their upbringing, a new report has revealed.
The study found that even at the early age gaps have begun to show in a child's health, depending on whether they are living in a well-off or poorer household.
Alarmingly youngsters from disadvantaged homes are already less likely to be considered very healthy, compared to kids from wealthier households.
The research for Growing Up in Ireland matches international trends and marks the earliest age at which poverty and privilege manifest in a child's well being.
It showed that diet is having an impact on three-year-olds, with children from poorer families more likely to be obese and those with less-educated mothers more likely to eat burgers and crisps than fresh fruit or vegetables.
"What this report highlights is that even by three years, there is evidence of an emerging gap in child outcomes relating to levels of social advantage," Children's Minister Frances Fitzgerald said.
The minister said while the importance of the first three years of life is well documented, the new report gives a "rich description" of the family, childcare and financial circumstances the country's youngsters live in.
Three-year-olds from more socially-advantaged backgrounds were more likely to be described as "very healthy" at 75% compared with 67% of those from less well-off families.
The report showed that social class plays a role in obesity levels among youngsters.
Weight problems affected around 5% of three-year-olds from the most socially-advantaged group, compared with 9% among the most disadvantaged.
Dr Aisling Murray, co-author of the report, could help the Government identify where money needs to be spent in service provision.
"This will allow us to investigate further how the earliest life experiences affect child outcomes in later years and underlines the very considerable potential for Growing Up in Ireland to provide input to the development of evidence-based policy and service provision aimed at children and families in Ireland," she said.
Almost a quarter of the children were found to be overweight and 6% obese.
Some 98% of three-year-olds were described as "very healthy" or "healthy", with girls marginally healthier than boys.
The three-year-olds were examined as part of wider research that began in 2006, involving two groups - 11,000 children who were recruited into the study aged nine months, and 8,500 who were recruited at nine years of age.
The study is expected to continue for a further five years from 2015.
Other findings among those now aged three included evidence that children from more disadvantaged groups were more likely to display behavioural problems.
Children being brought up by parents in some form of stress were also more likely to experience behavioural difficulties.
Elsewhere, almost two-thirds of the families with three-year-olds said the recession had had a big effect on their lives since their last interview when their child was nine months old.
Nearly a third said they had been forced to cut back on basics as a result, while 14% were behind with their utility bills and 9% with their mortgage or rent.
Meanwhile, 85% of the three-year-olds studied were in two-parent families.
Almost all of them were living with their biological parent or parents, in either one- or two-parent families.