Mouse age for hybridoma fusions?

[Allison]

In a project to make monoclonal antibodies the standard procedure is to 
start with mice 4-6 weeks old, do a second boost after about 3 weeks, 
then, if the titer is good, go ahead with a final boost and fusion. 
This means that the mice are about 10-12 weeks old at the time of fusion.

However, sometimes a fusion doesn't work, or the resulting monoclonal 
antibodies don't have quite the reactivity that one wants, and 
subsequent fusions are called for.

Since I usually immunize 5 mice at a time and only use 1 or 2 in a first 
fusion I always have "spare" mice that could potentially be boosted for 
a second set of fusions. It would save a lot of time if I didn't have to 
start from scratch with new mice but would only be worthwhile if I had a 
reasonable fusion efficiency.

So my question is: at what age are immunized mice too old to use for 
fusions?

Does anyone have practical experience with this?  TIA
Allison