My spd with some anxiety guy was on singulair around 3-4 (for over a year, fuzzy on the details).
We discontinued to rule it out for the behaviour issues (he looked ADHD, strong SPD stuff), and didn't notice a difference. For him, it was a developmental stage.
Where we noticed behaviour was when he was getting a lot of the other puffers. I think it was the mix, while he could manage the singulair on it's own. The singulair kept him healthy and out of hospital a lot as his reactive airways were usually viral induced.
And on a positive note, he appears to have entirely outgrown it, after a number of hospitalizations and scary early years - he's been med free for almost 2 years!!
Mom to a teenager and a middle schooler.