I think it depends on the wood to, not all osage is equal
I have 2 bows made 3 years ago fom 2 different trees cut within a month of each other ,1 started getting dark pretty good but the other still a brighter looking yellow I have a 2 year old stave on the wall from another tree & it looks 10
Year old dark all ready the ones that seem to darken quicker are the ones that have the dark brown redish rings when first cut at least in my case !