What are the current age rules for minor dependent ID cards?

Military ID cards are not typically issued for dependents under the age of 10. BUT there is still in effect a temporary rule set in 2019 during the pandemic that raised the age to 14.
"QUESTION 10: How old does my dependent have to be in order to be issued a
DoD ID Card?
Answer: Dependents of members and former members of the uniformed services are eligible for
benefits in accordance with Federal law. The definition of dependent is driven by Section 1072 of
Title 10, United States Code, and is codified in DoD policy at DoDM 1000.13, Volumes 1 and 2.
Dependents include a spouse, unmarried children under 21 (including wards and adopted
children), unmarried children aged 21-23 who are attending school as full-time students,
unmarried children over age 21 who are mentally or physically incapacitated and incapable of
self-support, and parents who are reliant on the sponsor for over 50% of their support.
In accordance with the Under Secretary of Defense for Personnel and Readiness Memorandum,
“Extended Policy Guidance for Identification Card Operations During the Coronavirus Disease
2019 Pandemic,” dated June 2, 2021, the minimum age for initial ID card issuance is increased
from 10 to 14. AFI 36-3026, which is a joint Service policy, sets guidelines for the issuance of an ID card to a child under age 10, which now should be considered for children under age 14. Chapter 4.4 and
Table 4.3 lists 5 instances where an ID would be issued to a child under age 10, which should now
be considered for children under age 14:

And there are exceptions to the age rule.
1) The child’s temporary guardian lives away from the parent's duty station and must use a
medical facility other than the one keeping the child's records; or the child resides
temporarily away from the sponsor and there is no eligible spouse.
2) The child is of a joint service couple or a single parent.
3) The child's physical appearance warrants issue
4) The child does not reside in the household of an eligible adult family member.
5) Under unique circumstances and the Super Verifying Official/Site Security Manager
authorizes.
References:
- https://www.cac.mil/Portals/53/Documents/DODM-1000.13_vol1.pdf
- https://www.cac.mil/Portals/53/Documents/DODM-1000.13_vol2.pdf"

You should contact your DEERS office and find out if that rule is still in place. If rescinded the min age should be back to 10 years old.