A man has three daughters. The product of their ages is $168$, and he remembers that the sum of their ages is the number of trees in his yard. He counts the trees but cannot determine any of their ages. What are all possible ages of his oldest daughter?
1. **Factorize the product of the ages:**
   \[
   168 = 2^3 \cdot 3 \cdot 7
   \]
   This means the ages must be factors of 168.

2. **List all possible combinations of ages:**
   We need to find all sets of three integers whose product is 168. Since one of the ages must be a multiple of 7, we start by considering the factor 7.

3. **Consider the factor 7:**
   - If one of the ages is 7, then the product of the other two ages must be:
     \[
     \frac{168}{7} = 24
     \]
     The pairs of factors of 24 are:
     \[
     (1, 24), (2, 12), (3, 8), (4, 6)
     \]
     This gives us the combinations:
     \[
     (7, 1, 24), (7, 2, 12), (7, 3, 8), (7, 4, 6)
     \]

4. **Consider the factor 14:**
   - If one of the ages is 14, then the product of the other two ages must be:
     \[
     \frac{168}{14} = 12
     \]
     The pairs of factors of 12 are:
     \[
     (1, 12), (2, 6), (3, 4)
     \]
     This gives us the combinations:
     \[
     (14, 1, 12), (14, 2, 6), (14, 3, 4)
     \]

5. **Consider the factor 21:**
   - If one of the ages is 21, then the product of the other two ages must be:
     \[
     \frac{168}{21} = 8
     \]
     The pairs of factors of 8 are:
     \[
     (1, 8), (2, 4)
     \]
     This gives us the combinations:
     \[
     (21, 1, 8), (21, 2, 4)
     \]

6. **Sum of ages:**
   The man cannot determine the ages even after counting the trees, which means there must be more than one combination with the same sum. We calculate the sums for each combination:
   - For (7, 1, 24): \(7 + 1 + 24 = 32\)
   - For (7, 2, 12): \(7 + 2 + 12 = 21\)
   - For (7, 3, 8): \(7 + 3 + 8 = 18\)
   - For (7, 4, 6): \(7 + 4 + 6 = 17\)
   - For (14, 1, 12): \(14 + 1 + 12 = 27\)
   - For (14, 2, 6): \(14 + 2 + 6 = 22\)
   - For (14, 3, 4): \(14 + 3 + 4 = 21\)
   - For (21, 1, 8): \(21 + 1 + 8 = 30\)
   - For (21, 2, 4): \(21 + 2 + 4 = 27\)

7. **Identify the ambiguous sums:**
   The sums that appear more than once are:
   - \(21\) from \((7, 2, 12)\) and \((14, 3, 4)\)
   - \(27\) from \((14, 1, 12)\) and \((21, 2, 4)\)

8. **Determine the possible ages of the oldest daughter:**
   From the ambiguous sums, the possible ages of the oldest daughter are:
   - For sum \(21\): \(12\) and \(14\)
   - For sum \(27\): \(12\) and \(21\)

   Therefore, the possible ages of the oldest daughter are:
   \[
   12, 14, 21
   \]

The final answer is \(\boxed{12, 14, 21}\)