My son is soon to be six years old and his speech resembles that of a much younger child. Most of his peers seem to be much more intelligible. Is this a developmental issue or something that will come out in the wash?
Thanks,
Rob
Have you had his hearing evaluated? Is his vocabulary progressing? Does he speak in complete sentences? Does he have good or poor articulation? Does he follow along when you read to him? Does he have normal interactions with other people? Are other skills, like drawing and reading, showing signs of development?
One symptom, a hundred different possible causes. Put your mind at ease and ask his doctor for suggestions.
Ask your pediatrician, not a bunch of yahoos on the internet. Even if it’s a great bunch of yahoos like us! 
Get a recommendation for a speech therapist from your pediatrician. If he does need speech therapy, now is the time to start. Preschool kids are tolerant of peers with non-standard speech patterns. Grade school kids will be crueler, which could affect his socialization. My daughter started speech therapy in preschool and it was tremendously helpful. She never would have grown out of it on her own.
Just ask his teachers what their off-the-cuff opinion is. Early childhood educators, especially ones with bachelor’s degrees, are extremely adept at spotting developmental pathologies. In fact, more developmental disorders are first identified by teachers than by parents or pediatricians.
Check with your school district to see what resources are available. I was concerned about my daughter’s speech development and scheduled a complete developmental evaluation through the early childhood folks in the school district. The speech pathologist we saw was very thorough and knowledgeable.
ETA: My concerns with my daughter’s speech centered on the fact that she wasn’t pronouncing some letters correctly and therefore sounded younger than she was. Her vocabulary and grammar were fine. The speech pathologist was strongly of the opinion that she would outgrow those speech patterns on her own. That was probably 18 months or so ago, and she is doing better, but still working on Ls and Rs.
And we have a winnah!
Speaking as a pediatrician we are not the best resource for this but your tax dollars have already paid for a speech evaluation using your local public school resources. Take advantage of it. Now if you feel they are blowing off your concerns then discussing getting a referral for a private eval from your pediatrician might be indicated. But take advantage of the school resources first.
The actual answer, which they will give you, depends on exactly which sounds your child is having a problem with and whether or not the issue is exclusively speech, that is clarity of the sounds, or language, that is the words used and the structures in which they are used. Many kids at 6 still have trouble with certain blend sounds for example, while trouble with the basic building block sounds is much less common. Of course if there are other concerns, social interactions being difficult, or anything that makes you worry, running it by your pediatrician is still a good idea.