My LO jus started walking independently (but still a bit wobbly) at 15+ months.. my mil said he is slow .. I mean , 15 months started walking independently really is considered slow ?

Don't compare. Each child grow at their own pace. I won't say slow or not. It's just that they develop differently. I feel that we should label them as slow or fast. My 10month old can't crawl till date. Lol.