Do you really want to reduce the life of your old laptops?
Why don't you sell them instead and buy coins with the money?
If you really want to mine, you can try Electroneum. I tried with an old i3 laptop, mining around 20-25 h/s. I don't think its worth it, but you can just try.
I'm GPU mining at home with 1.4Kh/s which is making approx 30 ETNs per day.
And it's perfect for the winter time, to heat up your house/room
i am newbie in this field but can you tell me where i can buy them. and second thing i want to ask what type of GPUs you are using.