Since the Neolithic period the native inhabitants of the Lithuanian territory have not been replaced by any other ethnic group, so there is a high probability that the inhabitants of present-day Lithuania have preserved the genetic composition of their forebears relatively undisturbed by the major demographic movements, although without being actually isolated from them. The Lithuanian population appears to be relatively homogeneous, without apparent genetic differences among ethnic subgroups.

A 2004 analysis of MtDNA in the Lithuanian population revealed that Lithuanians are close to the Slavic and Finno-Ugric speaking populations of Northern and Eastern Europe. Y-chromosome SNP haplogroup analysis showed Lithuanians to be closest to Latvians and Estonians.

According to 2014 estimates, the age structure of the population was as follows: 0–14 years, 13.5% (male 243,001/female 230,674); 15–64 years: 69.5% (male 1,200,196/female 1,235,300); 65 years and over: 16.8% (male 207,222/female 389,345). The median age was 41.2 years (male: 38.5, female: 43.7).

Lithuania has a sub-replacement fertility rate: the total fertility rate (TFR) in Lithuania is 1.59 children born/woman (2015 estimates). As of 2014, 29% of births were to unmarried women. The age at first marriage in 2013 was 27 years for women and 29.3 years for men.