I think the leader in mane6 is Twilight Sparkle because she's smart and good in magic.But how old are all of the mane6 ponies.Well,this my opinions 

Rarity-25 or 26(the oldest)

Applejack-24 or 25

Rainbow Dash and Fluttershy have the same age-22 or 23

Twilight Sparkle-20 or 21

Pinkie Pie-18 or 19(the youngest

Why Scootaloo doesn't related to Rainbow Dash.Maybe Rainbow Dash doesn't has any relative why not Scootaloo be her adopted sister or cousin or niece(maybe).

For the CMC's ages

Apple Bloom-8 or 9 or 10

Scootaloo- 10 or 11or 12

Sweetie Belle-7 or 8 or 9

Babs Seed -6 or 7 or 8 (she is smaller than others)  

The other ponies 

                                                                                                                                                                    Spitfire-32 or 33 (she's a captain)

Cheerilee-23 or 24

Zecora-40 or 41

Mrs.Cake-34 or 35

Mr Cake-37 or 38

Lighting Dust -20 or 21(she's more energetic than Rainbow Dash)