"Old brown" is added to fresh lith developer to speed up the ageing because the lith effect is more pronounced and considered better as the developer approaches exhaustion. Consequently, the age of the "old brown" is not an issue. I have had a 2.5 litre bottle of "old brown" for nearly 20 years that has never been emptied. Having mixed fresh developer and added the amount of "old brown" needed, usually 1 litre to 3 litres of working sol'n, I simply top up the "old brown" bottle with the near exhausted developer left in the tray at the end of each session of lith printing.