[ I decided in the wee hours of the morning to do math lolwut and I may have gotten some timelines situated for Break, Sheryl, and Shelly using what we know about Kevin and Rufus as the center axis -- with some assistance from Sharon and Liam, as well. ]
Most of this is close approximation, but a good deal of these ages or points on the timelines are canon or otherwise confirmed through other related media... I think. Pretty sure. Idunnomanit's5AMwhileI'mwritingthis.
SO YEAH. Take these calculations with a grain of salt, but they're probably really, really within the ballpark at the very least.
It's said that Rufus is born 50 years after the Tragedy of Sablier. Considering the Four Dukedoms were established at around the same time and the similarities in their appearances via flashbacks, Sheryl is most likely around Rufus' age.
Kevin is born ~42 years following the Tragedy. It's said by him that it takes ~10 years for the capital to be moved from the ruins of Sablier to Reveille. So, Reveille has only been the capital for ~32 years by the time he is born. When he is 24 years old, the Sinclair Family is originally wiped out, and Kevin becomes the Illegal contractor for Albus the White Knight and begins his campaign.
The "Red-Eyed Phantom" massacre begins ~66 years following the Tragedy. Rufus enters Lutwidge Academy around this time, so he is most likely ~16 when Kevin begins to kill people to feed his contract. Rufus' grandfather serves as Duke Barma during Kevin's rampage, and it is implied in the manga that Duke Barma is either killed by Kevin's own hand or dies not long before Kevin falls into the Abyss.
Kevin is 24 years old when he meets with the Will of the Abyss.
Meanwhile, Rufus contracts with Dodo at the age of 37, meaning Sheryl is most likely in her late 30s at this time. In Retrace 86, Reim makes mention that the letter he reads concerning Sheryl, Barma, and Shelly is about 40 years old and that Shelly would have already been 3 or 4 years old at the time. Sheryl was likely between 23 and 24 years old when she gave birth to Shelly.
When Kevin arrives at the Rainsworth Manor, Sharon is 8 years old and Liam is 11 years old. Kevin retains his 24-year old body as time passes strangely while in the Abyss.
Rufus is 52 when Kevin enters into the Rainsworth House, and Sheryl is likely in her early 50s. Shelly is likely between 28 and 29 years old. Shelly gave birth to Sharon when she was between 20 and 21 years old.
Kevin inquires after the Sinclair Family. Considering the Will of the Abyss granted his wish, the family lives an additional 4 years. The Sinclair Family is now wiped out 70 years after the Tragedy.
Kevin changes his name to Xerxes Break and remains in service to Rainsworth House for the next 15 years. He is 39 years old, Sharon is 23, Liam is 26, Rufus is 67, and Sheryl is in her late 60s.
Considering Sharon contracts with Equus at 13 and makes mention of how her mother is ill but still living at the time of Oz's First Coming of Age Ceremony, Shelly is between 33 and 34 years old at the time of Sharon's contract.
Shelly dies during the 10 years Oz spends in the Abyss. This makes her 44 at the absolute oldest but considering her deteriorating health, she most likely died in her mid 30s.
Finally, considering Rufus is ~16 when Kevin enters the Abyss and 52 when Kevin escapes, Break jumps ~36 years into the future. Technically, then, Break is ~75 years old.
(And approximately 8 years Rufus' senior, even though Rufus' physical body is 13 years older.)
