If he got a suspended sentence, then it suggests there's more to it than him 'thinking she was older', and he didn't do anything wrong. He must have been reported by someone too. With a suspended sentence I would imagine that the judge believed your boyfriend had prior knowledge, that he suspected she was underage or it was blindingly obvious.
Yes, some teens do look much older. My friend worked in a local hairdressers, along with another girl who was younger than us. This girl was a tall, willowy, pretty blonde and she was 15. She would get into clubs, drink in pubs with fake ID. A young lad used to come in the salon, he was around 20, training to be a PE teacher, and this girl fancied him and he liked her too - she'd told him she was 18. They went on a date, and she told him she was going on work experience the following week. He saw me and my friend in the street and asked us straight out 'How old is X, because she told me she's 18, yet she's going on work experience, it doesn't add up'. We were a bit hesitant, and he said 'Please tell me the truth, I'm training to be a PE teacher, it will destroy my career if she's underage'. My friend made excuses and left, but I told him the truth. He was very very thankful, but he said if she hadn't had made the slip up with the work experience, he may have never realised until too late. Nothing physical has taken place between them, because her slip up rang alarm bells for him. He was angry at the salon owner because she'd encouraged him to ask the girl out, knowing her employee was only 15!! This was back in the early 90s, when things like this happened a lot. Yes, it's not always obvious, as girls can appear mature. In the OP's situation, there's more to it, than her partner is saying.