Since at the time of Sonic Underground's creation and the fact they are animated in the style of the old 2D Sonic games. It is safe to say that Sonic, Sonia and Manic's age is 10 as they are triplets and Sonic's age was not changed to 15 until his character was redesigned, so only his redesigned form, as the one we know now, is 15 years old.
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Manic and Sonia are Sonic's brother and sister in Sonic Underground only.
Sonic's brother is "Manic" and his sister is "Sonia."
Sonia was Sonic's (and Manic's) sister in Sonic Underground. She did not appear in any games.
Sonic has a brother (Manic) and a sister (Sonia).
Sonic does not have siblings in the main-canon, but in the old DIC cartoon, Sonic Underground, he has a brother named Manic and a sister named Sonia.