age

No questions is silly (there are only silly answers... and I'll try to refrain from them )

Regarding the age, it depends on the chicken.

A commercial hybrid (especially one suce as ISA brown/warren) would be classed as "old" and may even be beyond laying.  A purebreed could be expected to live a lot longer.

With regards to egg laying, well again, it depends on the chicken.  Commercial hybrid layers (including blackrock, bluebelle, amberstars etc) are bred specifically to lay a very large number of eggs during their first season (upto 280!), now pullets are hatched with a set number of immature ova, just like a human female, and the more they lay the faster they run out... so while a hybrid layer might churn 6 eggs a week out for 80 weeks, that basically "empties them out."  A pure breed, while they may only lay 3 or 4 eggs a week, may do so for 4 or 5 years before dropping off.

Of course there are always exceptions to the rule.  some pure bred utility strains (and leghorns are a good example) will lay large quantitis of eggs, but again, thier stores will be depleted sooner.

As for mixing older and younger hens, well ther would be no reason not to... providing that any new birds were quarantined (in case they are carriers or harbouring any diseases/parasites) for  2 to 3 weeks before being introduced slowly to the  resident hens

HTH 

Hi.  Your Light Sussex are pure breeds, as is your Marans (always plural and with a capital M  

Your Black Rock is a hybrid (Rhode Island red x Barred Plymouth Rock... they are sex linked, making it easy to tell the boys from the girls on hatching, so the sellers can sell the girls only (most of the time!)

and all should continue laying for quite a few years (blackrocks have been known to still lay the occasional egg at the age of 8 - wonderful birds  ).

It's generally not recommended to mix ages of birds but as you aren't talking vast numbers here you can do it without too much risk.  

Basically the reason for not mixing ages is to do with disease.  Birds tend to be susceptable to diferent diseases at various ages hence for example older birds tend to pickup infectious bronchitis in older age so it's best not to mix them with younger birds etc.  Other examples exist.  The general recommendation is put your flock together with birds of similar ages and keep them together through life with minimum introductions.  Many hobbyist keepers ignore this advice but it's up to you what you do, but quarentine for 3 weeks will help though not entirely eliminate cross infection problems.

HF