I assume that a pony living in Equestria has a life expectancy equal to that of a human being living in the more developed areas of the planet Earth (75-80 years old). I also assume an Equestrian year is equal to that of an Earth year (365 days per year with each day 24 hours long and each year divided into 12 months).
I assume that an Equestrian pony turns adult at age 18. I assume that the Mane Six are young adults (18-25 years old). It must be noted that Fluttershy told Pinkie Pie in Griffon the Brush Off that she was a year older than Pinkie Pie.
I estimate the Cutie Mark Crusaders and other young ponies attending the Ponyville Schoolhouse are 7 to 12 years old, and the Mane Six were probably in the same age group when they first obtained their cutie marks.
I estimate that Big McIntosh is 5 to 10 years older than his sister Applejack, making Big Mac somewhere around 30 years old, and his younger sister Apple Bloom around 10 years old. Their grandmother Granny Smith must be nearing 80 years old, and their own mother and father probably 50 to 60 years old (if they are still alive).
Alicorns probably, in my estimation, live to be 2000 to 3000 years old. I estimate that Princess Celestia and Princess Luna were born as Alicorns (although their parents were likely ordinary unicorns) about 1200 years before the time of My Little Pony Friendship is Magic.
Dragons, Discord, and Lord Tirek probably have a life expectancy similar to that of Alicorns (2000 to 3000 years old). Discord and Tirek are likely at least 1500 years old at the time MLP:Friendship is Magic takes place.
As a baby dragon, I estimate that Spike is 10 to 12 years old.