Discovering the age of a mass relay is not an easy task. Relays can shield themselves to preserve their integrity down to the quantum level, so taking a sample for analysis proves nigh impossible. Relays also maintain self-cleaning cycles, wiping away potential evidence, though damage or dormancy can cause this cycle to break down (such as in the case of the Charon Relay, which became covered in ice).
A breakthrough was recently made by Dr. Amanda Kenson, who revisited old methods of dating the relays by testing dust trapped in their gravity wells. By comparing the relative velocity of relays to that of the stars they orbit as well as the composition of the dust around dormant relays against the dust at known locations in their star systems, Kenson could create a timeline of when a relay passed through the dust. The result dates the relay back millions of years; some may even predate the Protheans.
Kenson concluded that the oldest known relay is in batarian space. Dubbed "Alpha", it resides near the star Bahak and is unusual in its potential range and versatility. Alpha normally sends and receives mass at the range of a normal secondary relay, but if certain controls are adjusted, it becomes powered by an unprecedented amount of dark energy that could send cargo to sixteen other relays and even across a great distance to the Citadel. Hegemony authorities have kept this quiet, fearing retaliation from Council species who would assume hostile intent if they found a sudden new route into their space. It goes without saying that the Reapers have no such fear.