A method of estimating the epigenetic age of an individual, the method comprising: observing a presence or absence of methyl groups at a plurality of CpG markers that correlate with: counts of naive cytotoxic T cells in the population of leukocytes obtained from the individual; counts of exhausted cytotoxic T cells in the population of leukocytes obtained from the individual; and counts of plasma B cells in the population of leukocytes obtained from the individual; and comparing the methylation status of the plurality of CpG markers observed in the population of leukocytes from the individual with the methylation status of the same markers from an age matched reference population so as to obtain a value or a range of values for cell counts of naive cytotoxic T cells, exhausted cytotoxic T cells, and plasma B cells; and estimating the epigenetic age of the individual, wherein the plurality of CpG markers that correlate with counts of naive cytotoxic T cells comprises cg 15867698, cg 17478979, cg25289028, cg10909506, cg23001918, cg17820878, cg07107916, cg09025210, cg07899551, cg03370106, cg26485825, cg21097090, cg25130381, cgl6662477, cg06952412, cg05392293, cg26720010, cg04683740, cg08945443, cg20386303, cg15535471, cg25020550, cg24376214, cg21593149, cg25639084, cg15989436, cg02033323, cg18346531, cg02989940, cg10274029, cg07955474, cgl8442362, cg23876292, cg12966876, cg17850367, cg01372366, cg05913271, cg07094298 and cg10493055; the plurality of CpG markers that correlate with counts of exhausted cytotoxic T cells comprises cg07094298, cg04683740, cg26655856, cg14518178, cg01372366, cg09197075, cg03132824, cg00147638, cg25020550, cg23731272, cg22513455, cg00495443, cg08688907, cg21912203, cg10688297, cg10909506, cg26091609, cg00073460, cg08454507, cg20723792, cg14969094, cg01124420, cg06445016, cg05217983, cg03467087, cg02988775, cg01345395, cg16549957, cg21593149, cg00871371, cg11685391, cg23001918, cg26485825, cg08482359 and cg13608166; and; the plurality of CpG markers that correlate with counts of plasma B cells comprises cg24735235, cg01372366, cg25521400, cg01124420, cg13553498, cg26164712, cg20244489, cg08945443, cg13608166 and cg06245711. counts of naive cytotoxic T cells in the population of leukocytes obtained from the individual; counts of exhausted cytotoxic T cells in the population of leukocytes obtained from the individual; and counts of plasma B cells in the population of leukocytes obtained from the individual; and comparing the methylation status of the plurality of CpG markers observed in the population of leukocytes from the individual with the methylation status of the same markers from an age matched reference population so as to obtain a value or a range of values for cell counts of naive cytotoxic T cells, exhausted cytotoxic T cells, and plasma B cells; and estimating the epigenetic age of the individual, wherein the plurality of CpG markers that correlate with counts of naive cytotoxic T cells comprises cg 15867698, cg 17478979, cg25289028, cg10909506, cg23001918, cg17820878, cg07107916, cg09025210, cg07899551, cg03370106, cg26485825, cg21097090, cg25130381, cgl6662477, cg06952412, cg05392293, cg26720010, cg04683740, cg08945443, cg20386303, cg15535471, cg25020550, cg24376214, cg21593149, cg25639084, cg15989436, cg02033323, cg18346531, cg02989940, cg10274029, cg07955474, cgl8442362, cg23876292, cg12966876, cg17850367, cg01372366, cg05913271, cg07094298 and cg10493055; the plurality of CpG markers that correlate with counts of exhausted cytotoxic T cells comprises cg07094298, cg04683740, cg26655856, cg14518178, cg01372366, cg09197075, cg03132824, cg00147638, cg25020550, cg23731272, cg22513455, cg00495443, cg08688907, cg21912203, cg10688297, cg10909506, cg26091609, cg00073460, cg08454507, cg20723792, cg14969094, cg01124420, cg06445016, cg05217983, cg03467087, cg02988775, cg01345395, cg16549957, cg21593149, cg00871371, cg11685391, cg23001918, cg26485825, cg08482359 and cg13608166; and; the plurality of CpG markers that correlate with counts of plasma B cells comprises cg24735235, cg01372366, cg25521400, cg01124420, cg13553498, cg26164712, cg20244489, cg08945443, cg13608166 and cg06245711. the plurality of CpG markers that correlate with counts of naive cytotoxic T cells comprises cg 15867698, cg 17478979, cg25289028, cg10909506, cg23001918, cg17820878, cg07107916, cg09025210, cg07899551, cg03370106, cg26485825, cg21097090, cg25130381, cgl6662477, cg06952412, cg05392293, cg26720010, cg04683740, cg08945443, cg20386303, cg15535471, cg25020550, cg24376214, cg21593149, cg25639084, cg15989436, cg02033323, cg18346531, cg02989940, cg10274029, cg07955474, cgl8442362, cg23876292, cg12966876, cg17850367, cg01372366, cg05913271, cg07094298 and cg10493055; the plurality of CpG markers that correlate with counts of exhausted cytotoxic T cells comprises cg07094298, cg04683740, cg26655856, cg14518178, cg01372366, cg09197075, cg03132824, cg00147638, cg25020550, cg23731272, cg22513455, cg00495443, cg08688907, cg21912203, cg10688297, cg10909506, cg26091609, cg00073460, cg08454507, cg20723792, cg14969094, cg01124420, cg06445016, cg05217983, cg03467087, cg02988775, cg01345395, cg16549957, cg21593149, cg00871371, cg11685391, cg23001918, cg26485825, cg08482359 and cg13608166; and; the plurality of CpG markers that correlate with counts of plasma B cells comprises cg24735235, cg01372366, cg25521400, cg01124420, cg13553498, cg26164712, cg20244489, cg08945443, cg13608166 and cg06245711.