According to the Chronology Page, in 0069, Erio learned that he was an Artificial Mage and he was later taken away to be experimented on. Within the same year, the illegal facility which Erio was in was discovered and shut down by the TSAB, and Erio was taken to the Special Care Center.
Caro was banished from her tribe in 0071, so between 0071 and 0075, Caro enlisted with the TSAB. However, no unit wanted to employ her because she couldn't control her power yet. She met Fate one day by pure chance, and after hearing about Caro's past, Fate decided to take her in and became the her guardian.
In 0074, Erio left the Special Care Center and enlisted as TSAB private, so Fate met Erio in the Special Care Center between 0069 and 0074.
At the time Fate wanted to adopt Erio, she was too young to officially adopt him, so Erio was instead adopted by Lindy Harlaown. With Fate becoming his guardian, I assume this happened before Fate's 18th birthday. Since Fate was 15 in 0071, when Nanoha, Hayate, and Fate met in A's epilogue, and Fate was 19 when Riot Force 6 was formed in 0075, Fate would have had her 18th birthday in 0074. However, without an exact date on Fate's birthday, she could have been 17 between 0073 and 0074, which wouldn't be the case with Caro as she still had her family.
When Riot Force 6 was formed, and Caro and Erio met up to join the rest of the group, that was apparently the first time they met.
I'm not sure if it's included in the manga or not, but I'm wondering when Fate became Erio's and Caro's guardian. From the above, I assume it took place some time between 0071 and 0073/74 (before Fate's 18th birthday).