Mouse age for hybridoma fusions?

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