This cross-sectional study compared somatic, endurance performance determinants and heart rate variability (HRV) profiles of professional soccer players divided into different age groups: GI (17-19.9 years; n = 23), GII (20-24.9 years; n = 45), GIII (25-29.9 years; n = 30), and GIV (30-39 years; n = 26). Players underwent somatic and HRV assessment and maximal exercise testing. HRV was analyzed by spectral analysis of HRV, and high (HF) and low (LF) frequency power was transformed by a natural logarithm (Ln). Players in GIV (83 +/- 7 kg) were heavier (p < 0.05) compared to both GI (73 +/- 6 kg), and GII (78 +/- 6 kg). Significantly lower maximal oxygen uptake (VO2max, ml*kg(-1)*min(-1)) was observed for GIV (56.6 +/- 3.8) compared to GI (59.6 +/- 3.9), GII (59.4 +/- 4.2) and GIV (59.7 +/- 4.1). All agegroups, except for GII, demonstrated comparable relative maximal power output (Pmax). For supine HRV, significantly lower Ln HF (ms2) was identified in both GIII (7.1 +/- 0.8) and GIV (6.9 +/- 1.0) compared to GI (7.9 +/- 0.6) and GII (7.7 +/- 0.9). In conclusion, soccer players aged >25 years showed negligible differences in Pmax unlike the age group differences demonstrated in VO2max. A shift towards relative sympathetic dominance, particularly due to reduced vagal activity, was apparent after approximately 8 years of competing at the professional level.