I take a 7 month old, he likes: Duplo Train, Fairy Tale Brook and Explorers Train. He can't go on anything else, but he loves to people watch.
Last month I took an 11 month old, who also adores the Duplo Train and Fairy Tale Brook. She also liked watching the trains/boats/cars in MiniLand.
I would not take a child of this age to Legoland on their own - but with older siblings it is worth while (note: additional adult needed, as many rides do not permit a baby and many rides will need an adult to accompany a younger child). I meet up with several childminders, and all the children love going to Legoland - we do a few rides, play in the playground, watch Jonny Thunder show, build cars and towers.
We took 6yo DD and 18mo DS on Friday - he was particularly taken with the Balloon ride.
If DD hadn't been desperate to go though, we wouldn't have taken DS alone - even though he goes free it is a seriously expensive day out - I hunted out deals to get us all in as cheaply as poss., but it was still £105 for four adults, two kids & DS - plus parking, food & drink, souvenirs, etc.
Maybe wait until school hols are over if you can - it was pretty packed when we went and lots of littleys going into melt-down with the heat & sheer excitement.
Why too young? I took my DD about a month ago (now 7 months) she loved it. We went on rides like the boating school, balloon school, fairytale bookride, train, model village and watched the show they do. BUT we have annual passes to all the merlin attraction so didnt pay a penny - Under 3s are free anyway.