In this work, we test the hypothesis that narrow-line Seyfert 1 galaxies (NSL1s) are active galactic nuclei in their early phase and are therefore younger and more active than the more common broad-line Seyfert 1 galaxies (BLS1s). If that is true, then NLS1s should, on average, have lower black hole (BH) masses and higher accretion rates than BLS1s. To test this, we use a sample of 35 NLS1s and 54 BLS1s with similar X-ray luminosity distributions and good XMM-Newton observations. To determine the BH mass MBH, we apply an X-ray scaling method that is independent of any assumptions on the broad-line region dynamics and the inclination of the objects. We find that, on average, NLS1s have lower BH masses, but the difference between the average MBH of NLS1s and BLS1s in our sample is only marginally significant (at the 2.6σ level). According to a Kolmogorov-Smirnov test, the distribution of MBH values of NLS1s is different from that of BLS1s at the 99 per cent confidence level. Even stronger differences between NLS1s and BLS1s are inferred when the accretion rate distributions of NLS1s are compared to BLS1s, suggesting that the two populations are indeed distinct. Our study also indicates that the MBH values (both for NLS1s and BLS1s) determined with the X-ray scaling method are fully consistent with those obtained using reverberation mapping.