MLP:FIM S1 E1 & 2 = summer sun celebration before Luna return.
MLP:FIM S2
MLP:FIM S3
MLP:FIM S4 E1 & 2 = summer sun celebration after Luna return.
So S1, S2, S3 equal one year time in MLP:FIM.

RL Army/Reserve = 18 year minimum requirement.
So the Wonderbolt/Reserve would have an 18 year minimum requirement.

S4 E12 = Rainbow Dash celebrated her birthday and 1st year anniversary move to Ponyville.
S4 E21 = Rainbow Dash join Wonderbolt Reserve in making her age at least 18 years.
So Rainbow Dash would be 17 years old in S1 E1 - S4 E11.

Jim Miller on Spike: 10-12 dragon years but 16-22 pony years
10 dragon years x 1.6 aging rate = 16 pony years
12 dragon years x 1.84 aging rate = 22.08 pony years

So if 10-12 years pass before s1 e1 & 2 then Rainbow Dash would be 5-7 when she got her cutie mark.
7 + 10 year = 17 year old or 5 + 12 year = 17 year old.

I think Spike and Gabby are at least teenager or above being that both:

Have a job = 16 year minimum requirement

Both don't goes to school and able to travel without a guardian.

Work for their respected governments = 16 year minimum requirement

Gabby work for the post office = 18 years minimum requirement.

Spike serve as Equestria ambassador to the Changelings and Dragons = 21 years minimum requirement with college degree.

The smallest adult male human is 21.51 in which is 2.49 in below 2 ft (24 in).

Twilight and other have call Spike a baby dragon but in the secret of my success Twilight said ponies know nothing about dragon's biology. And dragon biology can't be compare to pony biology just like a dog biology can't be compare to human biology.

Spike travel to Canterlot alone to do royal business for Celestia. Spike act as an assistant, an adviser, a scribe, and a butler for the princesses.

The mane six pay Spike to watch their pets.

Nobody have told Spike it wrong for him to crush on Rarity. Nor have anybody told Rarity it wrong for her to encourage Spike's crush.

All of Spike friend are teenager or above ( mane six, Charity Sweetmint, Big Mac, Discord, Gabby, Ember, Thorax).

Spike is never shown going to school or hanging out with the C.M.C. or any other foal. And the C.M.C. never once ask Spike if he was Sweetie Bell's secret admirer.

Spike was able to convinced Fashion mogul, Hoity Toity, to come to Ponyville to see Rarity's fashion show.

The buffaloes tribe respect and listen to Spike.

The green dragon in the Everfree Forest tried to kill Spike.

Spike was allow to light the cauldron at the Equestria Game. He also was the narrator for a professional hearthwarming's eve play.

Spike is able to play various instruments ( piano, guitar, drum, trumpet) at a professional level.

All dragons that participate in the Gauntlet of Fire are at least teenager including Spike. Spike have been call a runt by both Ember and Garble.

I think that they’re both children. Ponies can get jobs once they get there cutiemarks so it could apply to dragons and Griffons. I always saw Spike as 12 after Molt Down and Gabby as 12 or 13.

Hmm... Seems pretty legit as far as how old the guys would be, well 'Spike' anyway. All I know for sure is that the age difference between a pony and a human is contrasting, as we've seen that their teenage appearance in the 'E.G.' series almost indicates the age length of how the ponies would be in their own world in Equestria. According to a chart, a three year old horse is near equivalent to an 18 year old teenager. How does that relate between how old Spike and Gabby would be? Not by much but it does add in a factor to take under consideration.

Personally I think Spike was somewhere around 14 to 15 when he arrived at ponyville and my best guess when it comes to Spike is by S9 before the final episode three to four have assumingly passed since Twilight arrived at ponyville and Spike's around 17 to 18 at her coronation by then.

You also need to bear in mind that as far as we can tell things work differently in equestria even possibly the age restriction of getting a job, working as a government representative, and able to choose not to go to school. And in Spikes case he lived with one of equestrias smartest unicorns and probably weaned some of things Celestia taught Twilight while also living with her as family. Add to the fact he was probably helping them both out even before the show which indicates his helpful personality in the show.

As for Gabby my best guess was that's she either older or younger than Spike by a year, but I'm going more on the former since Pinkie acts like that and she's older than Spike or Gabby.

It's funny we see how old and buff Spike becomes in the multi-something years by which the show would come to an end. You almost wonder how Gabby would turn out or if to this day they still keep in touch every so often. Ah well, in any case, these youngsters definitely have a bright future with Equestria bringing all the colonies together to maintain some form of peace and harmony. With any luck, it is sure to last for good while.