Hi G,
I am no expert, but just wanted to share my experience.
DD1 was 5.5 months old and she was already STTN since 4.5 mo. At your LO's age, she started to wake up at night for feeds, so the dr suggested introducing solids to her. I waited a week more and a few days before turning to 6mo, we started with fruit juice. Right after turning 6 mo, we started offering fruit and then veggies purees.
I guess there's nothing wrong to start before 6 mo. My nephew was BFed until 3 mo and at 4.5mo, his dr recommended starting with solids as the formula wasn't enough for him. He is fine (he's 2 1/2 yo now).
HTH