Glottochronology enables absolute dates to be estimated. Shared cognates (cognates meaning to have common historical origin) are calculate divergence times. However the method was found to be later discredited due to the data being unreliable. Due to this historical linguists have trouble with exact age estimation when pinpointing the age of the Indo-European language family. It could range from 4000 BP to 40,000 BP, or anywhere in-between those dates according to Dixon sourced from the rise and fall of language, (Cambridge University Press). As seen in the article here.