The term 'extended breastfeeding' seems to imply that it is longer than the 'norm'. So what is the 'norm' for breastfeeding? In Ireland, there are statistics available for breastfeeding initiation (55.6% in 2013). However, statistics are not routinely collated for breastfeeding continuation, never mind weaning age.
The World Health Organisation recommends that babies are breastfed exclusively for 6 months and continue to be breastfed up to 2 years and beyond.
Lawrence 1999 is often cited with the world average being '4 years', Informal research by Dr. Katherine Dettwyler, anthropologist at Texas A&M University, argues that the natural weaning age for human beings falls between 2.5 and 6 years of age: www.llli.org/ba/feb01.html.
Lawrence, R. A. and R. M. Lawrence. Breastfeeding:A Guide for the Medical Profession. St. Louis: Mosby, 1999.
Dettwyler, K. A. A time to wean. BREASTFEEDING ABSTRACTS 1994; 14:3-4.
Statistics for breastfeeding beyond initiation are difficult to get worldwide as they are not routinely collated. In this study of 179 breastfeeding mothers at a La Leche League Conference int he US, the average age of weaning was 2.5 years (3 years for the youngest child).
Sugarman M, Kendall-Tackett KA. Weaning ages in a sample of American women who practice extended breastfeeding. Clin Pediatr (Phila). 1995 Dec;34(12):642-7.