I'm settling this once and for all:

let's look at the episode cutie mark Chronicles; this was the episode where we saw when spike was first hatched so that will be our frame of reference, applejack said she was younger than applebloom at the time when these events occured and since twilight was doing a school entrance exam she might as well been the same age at the time because she hadn't started her primary education yet which puts her around the 4-5 year range, which sets the age gap between her and spike at roughly 4-5 years. By the events of the first episode twilight is a full grown mare, a student under celestia sure, but still fully grown. for the sake of this argument let's low ball it and put her in late teenage years of 16-18 which means spike is around 11-14 ish when they arrive in ponyville. Between that and the events of school raze several years have to pass with nine villain defeats, twilight becoming an alicorn, flurry heart's birth, the equestria games, rainbow dash becoming a wonder bolt, rarity opening new boutiques, the CMC getting their cutie marks, the friendship castle sprouting up, the school of friendship being built and going through a rather turbulent semester, along with all the other zany adventures the mane six went on means he have about a 5-7 year time frame for all this (again this is a serious low balling), which would put twilight about 21-25 years old now which would make spike around 16-20 years old. And this is a seriously low estimate, the age gap doesn't change but twilight could have been older when she left with spike for ponyville making him older and more than 5-7 years could have past during the events of the show which again would make all the characters older, in any case spike is very likely a young adult by now, not a child, not a teenager, and adult (at least by pony standards, by dragon standards he's still a child but come on they live for hundreds of years.).

Damn.... that some strong logic there

What the comic where the mane 6 travel 20 years into the past to the day Spike was hatch

thats true

There's no way Twilight was that young. She was nearing cutie mark age. Besides, five year old Ponies have a distinctive appearance.

It seems that in dragon standards, Spike is a young adult also. Reason for this is on the episode “Molt” Smolder realizes that Spike was going through some changes, she also told him that by time one of her older siblings got kick out of the nest and that it was around that time that he would ‘terrorized’ some other ponies.

There's also the IDW 20/20 comic that recently came out that takes place 20 years in the past, on the day Spike is born.
https://previewsworld.com/Catalog/NOV180610

Okay, I'll try to establish a timeline.
Spike was hatched on the first Sonic Rainboom, which was for Twilight's entrance exam, meaning it had be in the summer before the start of the school year. Twilight and Spike came to Ponyville on the Summer Sun Celebration (the first day of summer) (0). Spike's birthday was seen in Secret of My Excess. Heath's Warming Eve would be about a half a year since they arrived in Ponyville (0.5). Princess Twilight Sparkle was one year after Friendship is Magic (1). Then we have Hearthbreakers (1.5), A Hearth's Warming Tail (2.5), The Hearth's Warming Club (3.5), and Best Gift Ever (4.5).
The canon of the IDW comics have always been... iffy. But if 20/20 is canon, then we can round up to about 5 years, making Spike almost 15 when he and Twilight moved to Ponyville.

6762275
Nice work! There is also Jim Miller's twitter page where said he consider Spike to be 10-12 in dragon years but 16-22 in pony year.
If ten years pass from when Twilight got her cutie-mark to the first episode then according to the following calculation Spike's age would be;
10 dragon years x 1.6 aging rate = 16 pony years or 12 dragon years x 1.84 aging rate = 22.08 pony years.

But there also evidence from the show that show Spike being older despite his size, look, or how he acts.

1. He was in Canterlot on royal business which mean he was working for Celestia i.e. the government. Most governments required you to be 16 years old to work for them.

2. All the dragons that participated in the Gaunlet of Fire were teenagers or above which would include Spike. Remember Ember call Spike a runt which mean she consider him to be small for his age.

3. The Cutie-mark Crusaders only sought cutie-mark advice from grown ponies not from their classmates. Scootaloo got the zipline idea from Spike which mean Scootaloo consider Spike to be in the same age range as Rainbow Dash.

4.Hoity Toity only can to Ponyville because Spike tqlk to him. I doubt a fashion mogul would take time out of his busy schedule to follow a "kid" hunch.

5. We don't ever see Ms. Cheerilee trying to get Spike enroll at school. You think a dutiful teacher would want all the kids to be in school.

6.In the Break-up Break-down, when the Cutie-mark Crusaders were trying to find Sweetie Bell secret admirer, they only ask the colts in their class. Never once did they ask or thought that it could be Spike which mean they view Spike being above their age group.

6762632
like I said, I was seriously low-balling the math here for the sake of the argument to show that at the ABSOLUTE MINIMUM spike is a young adult in terms of age, there is a very good chance he's much older.