Improved between the ages of 2 months to 16 months. But look at the HbSS and that goes on from here to eternity.
Side effects later on are unfortunate but irrelevant. As long as you make it to the age where you can reproduce, it's a win from an evolutionary point of view - conversely, dying before then is a major loss. I've no doubt that mortality rates creep up faster in carriers than in non-carriers later on in life, but the goal is to pass on your genes, which you can't do if you die at the age of 14 months. And once you've done that, your part is played. Evolution is about survival, not long and happy lives.