If they are a pure breed and cheap ie a couple of quid, they might be OK.
Best (most productive) age to buy is about 18-22 weeks, called point of lay (POL). Most hens will start to lay 22-24 weeks. They lay best in their first year, then egg production decreases by about 25% each year. In the best year, the number of eggs may be between 260 and 300 eggs.
I haven't bought any birds this year but I was paying nine quid for POL hybrids last year. I sell my surplus at 1.50, so three dozen eggs pays for the hen (excluding feed costs of course).
Hope thsi helps, but others will have more information and views.