古老 (gǔlǎo) means 'ancient; age-old' and is the standard adjective for describing things that are very old, especially with historical or positive connotations. It is more common in everyday speech than 古 (gǔ), which is more literary and often appears in compound words (e.g., 古代). Compared to 悠久 (yōujiǔ), which emphasizes a long, time-honored duration (e.g., of culture, traditions), 古老 simply denotes great age.