Cleaning the mildew and mold from the wood is a good idea. You don't want it covered up. Cleaning can be accomplished in several ways. You may be able to remove the black stains using liquid dish soap and water and a scrub brush. Don't worry: the water is not going to harm the wood since it's able to dry rapidly. Chlorine bleach is also very effective at cleaning up the mold and mildew. You can mix a 50-50 solution with regular chlorine bleach and water. Some people don't do well with the fumes, so be careful.
A - Use the seat belt fit test on all children under 13 years of age to be sure they are big enough to safely use the adult seat belt without a booster seat. Have the child sit all the way back on the vehicle seat. naturally at the seat edge. If they do, continue the test.
Some go into the booster seat before they r 2 yrs old. Now the law is enforcing that kids be in booster seat til they turn 8 yrs old. Mine has long legs so I had to switch them around 15 or 16 months old.They weighed over 30 lbs too. Going by what your child can do is alot safer than going by age. If your child can sit without support, and hold his own head up, and is at least 24 pounds ( a little less is okay.) Then is is probably okay to put him in a booster seat.