score:9
I'd say William Hiseland, who fought in the Battle of Malplaquet at the age of 89.
The Scotsman newspaper writes about it:
At the Battle of Malplaquet, the regiment probably had the youngest and oldest participants on the battlefield. The wife of a soldier, Private McBain, handed over their three-week old baby son to him just before the battle saying she would follow the Colours no more. McBain placed the baby in his knapsack where he remained, and survived throughout the battle.
Also serving in the regiment as a soldier that day was William Hiseland, born in 1620, who, at 89, was almost certainly the oldest man on the field. Having survived the battle he lived until the age of 112, dying in 1732, as an in-pensioner at the Royal Hospital Chelsea, having got married at the age of 103.
Upvote:4
By making a search over some var memorial archives you can find remarkably old soldiers.
For example,
Drobyshev Illarion Pavlovich born in 1839 was killed in 1948 in combat with bandits in Tambov Oblast. HGe was deputy-commander of a company, and if the documents are correct he was 109 years old.
http://obd-memorial.ru/html/info.htm?id=9862711
Pichuzhkin Pavel Feodorovich born in 1833 was killed in 1944 near Veazma. According the documents he was 111 years old.
http://obd-memorial.ru/html/info.htm?id=261298833
Chontuvian Arony Philippovich born in 1832 deceased in 1943 of pneumonia, being a military sailor. According the documents he also was 111 years old.
http://obd-memorial.ru/html/info.htm?id=3293042
Dolgopolov Philipp Ivanovich born in 1831 was killed in 1943 in Leningrad area. He was 112 according the documents.
http://obd-memorial.ru/html/info.htm?id=261023681
Semeanin Ivan Ivanovich born in 1830 was killed in 1943 near village Scherbinka (near Novgorod area) being a private. He was 113.
http://obd-memorial.ru/html/info.htm?id=260835113
Proseanykh Illarion Kanafievich born in 1829, was killed in 1945. The cause of the death is unknown, he was a private. He was 116.
http://obd-memorial.ru/html/info.htm?id=269053989
Kovalchuk Zakhar born in 1828 died in a German POW camp in 1944, at age 116.
http://obd-memorial.ru/html/info.htm?id=78438709
Tonkikh Matvey Gordeyevich of 1828 was killed in 1943 being a private at age 115.
http://obd-memorial.ru/html/info.htm?id=260553416
Gavrilov Ivan Dmitriyevich also of 1828 was killed in 1943 at age 115.
http://obd-memorial.ru/html/info.htm?id=269058043
Zhukov Ivan Frolovich of 1827 died in German POW camp in 1945, although he was captured in 1941.
Volkovich Peter Vasilievich of 1827 was killed in 1944 in Belarus at age of 117 being a partisan.
http://obd-memorial.ru/html/info.htm?id=261841106
Some private Ivanov F.F. of 1826 was killed in 1944 near Tver at age 118.
http://obd-memorial.ru/html/info.htm?id=261447957
Timofeyev Konstantin Filippovich of 1825 died of wounds in 1944 at age 119
http://obd-memorial.ru/html/info.htm?id=55883101
Nikorenko Pavel Denisovich of 1824 was killed in 1845 in Kuybyshev area.
http://obd-memorial.ru/html/info.htm?id=4749823
....
There are lots of similar records, but most of them seem to be just typos.
The oldest record I found is Ivanov Vladimir Ivanovich, a warehouse guard who died in 1942 and according the documents was born in 1778!