It appears to me that here there are two questions. It is possible that the APM has arms too narrows for exploration. We have used in rats, an APM having arms appropriate to the size of the animals and rarely fall down. The second point is that it is not advisable to put edges to the open arms, since this arm is an adequate stimulus for testing the conflict of motivation-fear to exploration that this device is pretending to measure. The addition of borders will decrease the impact of the environmental conflict. Perhaps, you should try an APM adapted to rats instead for mice.

Normally, the mice should not fall off the EPM. The mice you are using are not aging. The presence of additional deficits, such as locomotion or vision problems, in mice is a possibility. The locomotion of the mice can be assessed in an open field setting, and there are multiple tests that can be utilised to ascertain any potential difficulties they may encounter in perceiving their environment (a scenario that may be more likely if the mice are of a white genetic background).