Changing the tables to include older pupils could encourage schools to hold pupils back deliberately. The risk then, according to education minister Baroness Blackstone, would be disaffected 16-year-olds quitting school without having seen an exam paper.
This year 32,000 pupils in England left school with no GCSEs or GNVQs, the minister added. And she would oppose changes which could cause this figure to rise.
The league tables currently show the GCSE and GNVQ perfomance of 15-year-olds and A-level results of 17-year-olds.
The results of pupils who take GCSEs early are eventually counted when they reach 15. But the results of those who take exams later are never included. Those who take GCSEs a year later than normal are counted as having no exam passes.
Headteachers have argued for all pupils' results to be counted whatever their age.
John Dunford, general secretary of the Secondary Heads Association, said:
"League tables based on age have done immeasurable damage. The sooner we depart from age-related qualifications the better."