For what it is worth, here is James Dixon’s service career of the Enterprise
2218
16–Jul–2218
The Enterprise is laid down at UESPA in San Francisco Yards on Earth
2219
July
Robert April is chosen to be first Captain of the Enterprise
2220
March
Free-space assembly starts on the Enterprise
16–Jun–2220
A christening projectile is perfectly aligned and fired to intercept the Enterprise in exactly 2 weeks, 4 days, and 16 hours
4–Jul–2220
The USS Enterprise (NCC-1701) is launched from Earth
2221
5–Jan–2221
The USS Enterprise (NCC-1701) heavy cruiser is commissioned
2244
2–Oct–2244
Robert April announces he is stepping down as Captain of the Enterprise and names Christopher Pike as his successor
2245
April
The Enterprise is recommissioned with Captain Christopher Pike in command
2247
June
Enterprise begins its first Five–Year–Mission under the command of Captain Christopher Pike with Spock as his Science/Second Officer
2252
June
Enterprise's first Five–Year–Mission under Captain Pike ends. A second Five–Year–Mission under the same commander will begin next year
2258
April
Enterprise returns from its second Five–Year–Mission. Captain Pike is promoted to Fleet Captain. Commander James Kirk is promoted to Captain and given command of the Enterprise
2259
June
Where No Man Has Gone Before
2260
4–Jan–2260
In the Gamma 400 system, in orbit of Starbase 12, Enterprise is drydocked and converted over to a Bonhomme Richard–class heavy cruiser configuration
4–May–2260
Enterprise is relaunched, following Bonhomme Richard–class refitting
YEAR ONE
9–Jun–2260 to 8–Jun–2261
9–Jun–2260
The USS Enterprise is recommissioned after her uprating to the Bonhomme Richard–class specifications. The 3rd Five–Year–Mission of the Enterprise officially begins
YEAR TWO
9–Jun–2261 to 8–Jun–2262
YEAR THREE
9–Jun–2262 to 8–Jun–2263
YEAR FOUR
9–Jun–2263 to 8–Jun–2264
YEAR FIVE
9–Jun–2264 to 8–Jun–2265
16–Jun–2264
The USS Enterprise enters the Arias Mastac Prime Dockyards in orbit of Daran V and is uprated to the Achernar–class configuration
3–Oct–2264
The USS Enterprise is relaunched, now converted to the Achernar configuration
5–Nov–2264
The USS Enterprise is recommissioned as an Achernar–class heavy cruiser and continues her Five–Year–Mission under Captain James Kirk.
2265
March
The Five–Year–Mission of the USS Enterprise finally ends
2266
3–Jan–2266
The USS Enterprise enters Drydock Number One in orbit of Earth and uprating begins to form the new Enterprise (NCC-1701) class starship
2267
23–Aug–2267
THE MOTION PICTURE
2268
17–May–2268
The USS Enterprise (NCC-1701) heavy cruiser is recommissioned. The first of two Five–Year–Missions of the new USS Enterprise begins with Admiral James Kirk in command
2273
March
The first Five–Year–Missions of the upgraded USS Enterprise ends. A second Five–Year–Missions, also with Captain James Kirk in command, will soon begin this year
2278
April
The Enterprise returns from her final Five–Year–Mission. The Enterprise is drydocked for damage evaluation and refitting, following her last deep-space mission, and undergoes a refit which will last anywhere from six months to a year. Following the refit, the Enterprise is converted into a training vessel
2287
28–Jan–2287
THE WRATH OF KHAN
1–Feb–2287
THE SEARCH FOR SPOCK
Destruction of the Enterprise
25–Feb–2287
Funding is secured for an additional heavy cruiser of Levant specifications
28–Feb–2287
The Levant–class is rechristened Enterprise (II) class and the Enterprise–A is inserted into the production schedule at Starfleet Division's San Francisco facility.
1–Mar–2287
Construction begins on the Enterprise (II)–A (Ex–Levant NCC–1843) heavy cruiser at Starfleet Division, San Francisco Yards, Earth
APRIL
THE VOYAGE HOME
2288
17–May–2288
At his trial, Admiral Kirk is demoted to Captain rank and is given command of a new Enterprise (NCC–1701–A) which is launched from Spacedock
7–June–2288
THE FINAL FRONTIER
2289
30–Jan–2289
The uprated USS Enterprise (NCC–1701–A) heavy cruiser is commissioned
2291
15–Aug–2291
THE UNDISCOVERED COUNTRY
Enterprise–A decommissioned
2293
6–Jun–2293
GENERATIONS
Enterprise-B commissioned
Under this chronology, Enterprise is forty years old when Kirk assumes command, as per 'The Making of Star Trek'; and twenty years separates 'The Motion Picture' from 'The Search for Spock.' However, Enterprise-A is given a service life of only 2-3 years before being withdrawn from service and Enterprise-B taking its place.
With time, somewhere in my notes, I could probably dig up FASA's service life of Enterprise.