is it true that cargo carriers are less picky with age limits?
Not really. I fly 744's and 748's which cost a LOT of money to buy and insure, so why would they drop the requirements? For any modern airline it makes very little difference whether you're carrying passengers or cargo. They want pilots who stick to SOP's and experienced pilots so they can cut down on insurance and training expenses.
Of course you can think of joining some kind of cowboy outfit in Africa or Asia who will hire P2F/P4T lowtimers, but then again, it won't make much difference of whether it's a cargo or passenger outfit.
Thinking that you can 'jump the queue' and join a freight outfit in order to gain flight hours, well...let me put it this way: you won't be the only one who has this 'brilliant' idea. As covec put it, even during great economic boom years, the requirements for cargo airlines are still fairly high.
Past 30 and want to start flying? Really, you have to face reality and realize that the market is soooo saturated at the moment that airlines can pick and choose as they please. With the economy barely improving, it will probably only be worse in 2-3 years time because idiots will continue to start with flight training, thinking that they can beat the odds by desperately paying for type ratings and flight hours.
Sorry that I can't be more positive, but the only thing you'll be training for is to become a P2F slave, irrespective of whether you have cargo or in passengers in the back.