Once he turned 20 and became a full citizen, Agesilaus was elected to a common mess, presumably that of his elder half-brother Agis II, who had become king in 427, of which Lysander was perhaps a member.
The passage describes Agesilaus being chosen for a common mess after turning 20 and becoming a citizen. This suggests he was part of the military service structure under his half-brother Agis II, who became king in 427. Lysander's potential role within this system is implied.