On Junos EX platforms, mixed-speed AE configuration is not possible. 
Adding a non-existent mge interface in an AE of a different speed will throw a configuration commit error.

When a mge IFD supporting different default speeds in different platforms is added to an AE, 
which is already set on a particular speed by adding a different interface in AE first. 
The later port will be a non-existing IFD and will throw a configuration commit error as below.

error: Interface aeX with child links of mixed speed but link-speed mixed is not configured
error: configuration check-out failed

This issue might be seen if the following conditions are met:
* On Junos EX platforms
* Adding a non-existent mge interface in LAG with a mge interface of different speed

Mixed-speed LAG is not possible. So if the user wants to form a LAG with 10G of the 24mp, the other child member should also be a 10G port. 
With 2.5G mge ports, 10G mge ports wouldn't form LAG.
So 10G mge ports have to be configured for 2.5G speed via CLI. 
To make it the same, the mge interfaces can be configured for speed while adding it to LAG.

This issue is fixed in 22.3R2-S2.