To add Birth Year to your Live Entry template :

1. Open Live Entry and select the activity you wish to add the Seen trait to.
2. Select Setup > Data Entry Template to edit the data entry template. 
3. Find the Birth Year section on the Available Options list (it's towards to bottom of the list) and tick it.
4. Select the OK button. 

This will place the Birth Year on the data entry panel.

If you do not wish to assign a birth year to an animal, right click on the Birth Year in the Data Entry section to grey it out (disable it).