Competition was recognized as an important factor influencing the growth and development of Korean pine (Pinus koraiensis) trees, but the response mechanisms of young and middle-aged P. koraiensis under the canopy to competition remained poorly understood. This study focused on 15-year-old (young), 30-year-old (young), and 45-year-old (mid-aged) P. koraiensis trees in a mixed broadleaf-Korean pine forest at Maoershan Experimental Forest Farm of Northeast Forestry University, which is located in Shangzhi City, Heilongjiang Province, China. The Hegyi Competition Index (HCI) was used to quantify neighboring tree competition intensity, which was classified into five levels (HCI ranges: 0—5, 5—10, 10—15, 15—20 and 20—25). We analyzed the diameter and height growth, and physiological characteristics of young-to-mid-aged P. koraiensis trees under these competition intensities and explored their ecological adaptation mechanisms through correlation analysis. The results showed that (1) Neighborhood competition had a significant negative effect on the diameter and height growth of young and middle-aged P. koraiensis trees, and this inhibitory effect became more pronounced with increasing tree age. (2) Neighborhood competition intensity followed a power function relationship with both the diameter and height growth of young and middle-aged P. koraiensis trees. (3) The non-structural carbohydrates (NSC) and total nitrogen (TN) content in needles of the young and middle-aged P. koraiensis trees showed a trend of first increase and then decrease with increasing competition intensity. The NSC and TN content in needles of the 15-year-old P. koraiensis trees peaked at competition intensity level 3 (HCI 10—15), while those of the 30- and 45-year-old trees peaked at level 2 (HCI 5—10). The NSC content in needles was significantly positive correlated with TN content. The starch and total carbon content in fine roots remained relatively stable across competition intensities. (4) The gibberellin (GA) content in apical buds of the 15-year-old P. koraiensis trees was relatively stable, while that of the 30- and 45-year-old P. koraiensis trees gradually decreased with increasing competition intensity. The auxin (IAA) content in apical buds was significantly positive correlated with diameter and height growth. (5) The tolerance ranges of P. koraiensis for neighborhood competition intensity differed among age classes: 15-year-old trees tolerated intensity levels 1-3 (HCI 0—15), whereas 30- and 45-year-old trees tolerated only levels 1-2 (HCI 0—10). In conclusion, to ensure the normal growth and development of P. koraiensis trees under the canopy, the study determined that competition intensity for 15-year-old trees should not exceed 15 (HCI), while for 30- and 45-year-old trees, it should not exceed 10 (HCI).