Seeing as there has been quite a few time jumps, I’ve devised this table of comparative ages for characters with year 0 being the beginning of season 1. To clarify, 15 is considered a legal adult.
Twilight: 15; The Summer Sun Celebration was to be her first responsibility as an adult.
Pinkie Pie: 14; When she was 13 her parents decided that her creativity was being stifled living in the middle of nowhere and sent her to live in Ponyville. Still being a minor, she moved in with the Cakes, Carrot being Mrs. Pie's cousin.
Rainbow Dash: 15; Her father pulled some strings to get her on the weather team in Ponyville.
Fluttershy: 15; She moved to Ponyville at 14, though nopony knew she was a minor at the time.
Rarity: 16; Originally from Fillydelphia, she moved to Ponyville to start her boutique in a place without competition.
Applejack: 18; The most mature of the mane 6, both physically and mentally.
Midnight Star (David Corbin): 15; Forced into the military at 8, he was put into the mage guard at 12, and went AWOL at 14 when he learned Sarah was pregnant. At the time of Nightmare Moon's return, he was casting the spell that would separate the demon's blood from his body. Sarah was 16, Tiffany was 8 months.
CMC: 6; Sweetie Belle (6.5 yo) is the oldest by 6 months, Applebloom was second, followed two months later by Scootaloo.
To determine the age of an individual at the time of an event, just add the modifier listed to their previously mentioned age.
+0 Nightmare Moon's return. David Corbin's wife and daughter are killed by the demon, Nag-Ta.
+1 Discord's return.
+2 The royal wedding, changelings attack Equestria.
+2.5 The Crystal Empire returns.
+3 “Act I: The Broken Soldier”
+4 Midnight and Twilight are wed.
+4.5 Aurora is born.
+6 “Act II: The Greatest Crusade”
+6.5 “Young Love”
+7 "An Elegant Engagement"
+8.5 “Act III: Shadows of the Past”
+1384 Celestia finally admits she has an addiction to cake when her throne breaks under her extreme weight. Luna laughs.