It is very easy for little babies to get eye crossed when they are very small, like 3months old. They are too small that vision system is not fully developed. But when they are 7-8 month old, their vision system development can effectively prevent the crossing eyes. During this period, babies can move their eyes to see what is moving. If babies still have crossed eyes when they come to 7-8 months old, you should take it seriously. Take the babies to hospital for further examination as soon as possible. Usually adjusting the balance of muscles which support rotating eyes could help with crossed eyes. Further more, it is much easier for little kids to correct their crossing eyes. Patching or special glasses could be very helpful. Good luck.