Most children master the “th” sound between ages 5 and 7, making it one of the last speech sounds to fully develop. There are actually two versions of this sound, and each follows its own timeline. The voiceless “th” (as in “thumb” or “thin”) typically arrives first, around age 6, while the voiced “th” (as in “this” or “that”) often isn’t fully mastered until closer to age 7.
Why “Th” Takes So Long
The “th” sound belongs to a group called fricatives, which as a category are the last type of consonant children acquire. On average, all fricatives are acquired by age 6 years and 11 months using a 90% mastery criterion. Compare that to simpler sound types: most children nail down sounds like “p,” “b,” “m,” and “n” before age 4, and sounds like “ch” and “j” by about age 5.
“Th” is particularly tricky because it requires a tongue position that’s unlike almost any other sound in English. Your child has to place just the tip of the tongue between the upper and lower front teeth, then push a gentle stream of air over it. For the voiceless version (think “three”), there’s no vibration in the throat. For the voiced version (think “the”), the vocal cords hum at the same time. That combination of precise tongue placement and airflow control is a lot to coordinate, especially for a young child whose mouth is still growing.
Two “Th” Sounds, Two Timelines
Parents often don’t realize there are two distinct “th” sounds, but the developmental gap between them matters. The voiceless “th” in words like “think,” “bath,” and “three” is generally mastered by about 5 years and 11 months. The voiced “th” in words like “this,” “the,” and “brother” takes a bit longer, with mastery typically by 6 years and 11 months.
This makes sense when you consider that the voiced version requires everything the voiceless version does, plus the added step of engaging the vocal cords simultaneously. It’s a small difference in production but a meaningful one developmentally.
Common Substitutions Before Mastery
Before age 6, it’s completely normal for children to swap “th” for an easier sound. The most common substitutions are “f” for the voiceless “th” (“free” instead of “three,” “baf” instead of “bath”) and “d” for the voiced “th” (“dis” instead of “this,” “da” instead of “the”). You might also hear “v” substituted in some positions.
These substitutions aren’t errors in the clinical sense. They’re a predictable stage of development. A 4-year-old saying “I fink so” instead of “I think so” is doing exactly what you’d expect. Even many 5-year-olds are still working on it. The key question isn’t whether your child makes these swaps, but whether they persist well past the expected age range.
Dialect Matters
One important consideration: some patterns that look like “th” errors are actually features of a child’s home dialect. In African American English, for example, substituting “f” for “th” at the end of words (like “baf” for “bath”) or “d” for “th” at the beginning of words (like “dis” for “this”) are standard features of the dialect, not signs of a speech disorder. These patterns can look identical to developmental substitutions on paper, which is why context matters so much. A child who consistently uses these patterns in a way that matches their family’s speech community is speaking their dialect fluently, not struggling with articulation.
When Substitutions Might Be a Concern
Since “th” is a later-developing sound, speech-language pathologists generally don’t flag it as a concern before age 6. A child who can’t produce the voiceless “th” at age 5 is well within normal range. By age 7 or so, if both “th” sounds are still consistently absent or replaced, that’s when it becomes worth looking into.
That said, the “th” sound rarely exists in isolation. If your child is also struggling with other sounds that should already be in place for their age, or if their speech is difficult for unfamiliar listeners to understand, those are stronger signals that an evaluation could help. A child who’s only missing “th” at age 6 is in a very different situation from a child who’s also dropping other later sounds or having trouble being understood overall.
How “Th” Is Practiced
Whether at home or in therapy, the starting point for “th” is always tongue placement. The tip of the tongue should just barely peek between the front teeth. A common mistake is sticking the tongue out too far or pressing it too hard against the teeth. The goal is a relaxed position where air can flow between the tongue and the upper teeth, creating that characteristic soft friction.
Practice typically starts with the sound in isolation, then moves to syllables (“tha,” “tho,” “thee”), then to the beginning of words, and finally to conversation. The voiceless version is usually taught first since it’s simpler. Once a child can reliably produce the sound on its own, adding voicing for the “this/that” version becomes much easier because the tongue position is identical.
Many children pick it up quickly once they understand where their tongue goes. For others, especially those with tongue placement difficulties that prevent the tongue from extending far enough between the teeth, it takes more structured practice. The fact that “th” requires a tongue position found in very few other languages helps explain why it’s challenging not just for children but also for many adults learning English as a second language.