I know SIDS recommend not using second hand mattresses but is it really unsafe to use the mattress you've used for your first born for for your second child?

We have a really good quality inner-spring cot mattress for our Boori cot which we bought brand new with our cot. DD has been sleeping on it since she was 10 weeks old. She's still using it now (in her toddler bed) but she'll be in a normal single bed before our next baby arrives in late Dec so the mattress will have had a bit over 2 years use. Do I really need to buy a new mattress for #2?