plexi or uv plexi (basically the same) will block 
about 99.9 of UV light. It is significantly better 
than glass for this reason alone. It also will 
not break, but you may need the store to cut it 
for you unless you had a really good table saw 
available to you. If you use plexi, you will not 
need more than one layer.

Plexi as far as I know does not break down or 
deteriorate with time. My family has painting that 
have been framed in UV plexi for over 20 years and 
there is no deterioration that I know of. I know 
of nothing stating that plexi breaks down with 
age. 

Regardless of wether you use glass or plexi, the 
cels should not be placed in direct sun light. 
While plexi block most UV and UV glass blocks most 
UV, they do not block all UV and cels or prints 
will fade if left in the sun for a long time.

Catch you later,
Marcus