The oldest Cyrus child is Trace Cyrus. But according to Wide Open Country, he was born Neil Timothy Helson.
Trace is actually Tish’s son from her first marriage to Baxter Neal Helson. However, when she married Billy Ray, he adopted her son as his child, and they officially changed his name to Trace Dempsey Cyrus.
Source: https://www.distractify.com/p/miley-cyrus-siblings