What is the average officer career lifespan in the Army?

There are Mandatory removal dates, like enlisted RCP. At present, if you are under 55 years old (over 55 requires SECDEF waiver), the following ranks have the following years of service last I checked.
LTC = 24 years
COL = 30 years
GOs may serve up to 40 provided they keep getting nominated to a position at current or next grade. GOs can be out on the bricks in a month if they do not have a nominative position to go to at current or next grade.

Majors and below are murkier as they have to be selected for continuation despite being passed over for promotion (SELCON). With our draw down and withdrawal from Afghanistan and Iraq/Syria, I would expect SELCON to be drying up. If you are passed over in the Primary and Above the Zone then you must retire if eligible, separate or transition to the Reserve Component if they'll take you. If SELCON is granted, there is an MRD assigned to you and your file may be re-competed.

I can't get acces to the MRD calculator to verify, CAC enabled.

DA PAM 600-3 will tell you that a successful Army career is LTC with 20+ years of service.

Not sure what your question actually is. As SFC (Join to see) said, many separate after their initial ADSO. If you stay until 8 years under the old retirement system, you were foolish to leave, although there were some 14 and F-it people out there. The hard cut is MAJ to LTC. Once you make it to LTC, you can retire without SELCON. During the wars Majors could SELCON and retire. As could Captains with prior service. This new retirement system, I just don't know. I retired with the High 3 system and I'm happy about that.

In the Active Component, your year group would see:on, average:
90% still in at year 4
50% still in at year 8
40% still in at year 12
30% still in at year 20
12% still in at year 25
2% still in at year 30

You shouldn't consider what the average is, because that doesn't necessarily apply to you. Many officers choose to leave after they complete their ADSO and pursue employment in the civilian sector. For most officer branches, as long as you choose to stay, you can make it to retirement at 20 years. Depending on your branch promotion to LTC may be extremely competitive. Every promotion after 20 years becomes significantly more competitive. You get looked at for promotion two times for each grade, if you are passed over the second time, you will separate from the Army. If you are over 20 years you will retire. If you are under 20 you can request to stay on until retirement, but you will be ineligible for promotion. How long you can stay depends on how well you can promote, a lot of that depends on how competitive your branch is. Outside of a few re-branching opportunities early in your career, you don't get to choose what branch you get.