Juvenility in xylem formation can be looked as a mere biological ageing process. It can be also the result of biomechanical adaption to growing conditions different in the young ages. The field of structural mechanics regards resistance to external forces and posture control in long lasting progressive loading, according to external conditions as wind action or light availability. Many adaptive parameters are involved: inertia of the stem section, slenderness of this stem, position and weight of crown, basic wood density, grain angle, MFA, chemical composition of fibre wall affecting resistance to bending, elastic buckling or genesis of regulating forces (growth stresses). Besides the well known typical radial pattern of variation (TRP) in juvenile wood: decrease of ring width and MFA, increase in basic density with the distance to pith, there are other typical patterns associated to juvenile growth regarding pertinent mechanical parameters, common in primary or secondary tropical forests. One is the exact reverse of TRP for semi-tolerant emerging trees. Examples of typical transition from juvenile to mature xylem mechanical strategies are described with their implication for wood properties.