A long juvenile period limits the breeding process of many woody plants including tree peony. To investigate the physiological and transcriptomic differences between juvenile and adult plants of tree peony and to explore the keySQUAMOSA PROMOTER BINDING PROTEIN-LIKE(SPL) genes, which are vital in age-dependent pathways, 1-year-old and 3-year-oldPaeonia delavayiplants were used to compare the relevant physiological parameters and transcriptomic profiles of the leaves in two phases of plants. The results of the physiological parameters showed that the starch content in the leaves of adult plants remained unchanged and that the soluble sugar content significantly increased compared with those in the juvenile plants. In terms of plant hormones, the contents of cytokinin-like hormone (N6-isopentenyladenine (iP)) and jasmonic acid (JA) significantly decreased, whereas the contents of auxin (indole-3-acetic acid, IAA), abscisic acid (ABA), cytokinin-like hormone (N6-isopentenyladenenosine (iPR)), and ethylene precursor (1-aminocyclopropane-1-carboxylic acid, ACC) showed no statistic difference. Transcriptome sequencing results showed that there were 194 differentially expressed genes (DEGs) between juvenile and adult plants, including 171 up-regulated DEGs and 23 down-regulated DEGs. Circadian rhythm, plant hormone signal transduction, and sugar metabolism were closely related to the juvenile-to-adult transition inP. delavayi,involving a total of 12 DEGs. In addition, a total of 13SPLgenes were identified in the transcriptome data, but onlyPdSPL10(c71307.graph_c0) was differentially expressed. It was further validated via qRT-PCR analysis, indicating thatPdSPL10might be a key gene regulating the process of juvenile-to-adult inP. delavayi. Based on the above results, a hypothetical transcriptional network regulating juvenile-to-adult transition and flowering inP. delavayiwas proposed. These findings provide a reference for understanding the mechanism of juvenile-to-adult transition in tree peony.