The specific critical observation attributing “signs of adolescence" to Shelley's lyrics was made by T.S. Eliot.
T.S. Eliot, a prominent modernist critic and poet, discussed Percy Bysshe Shelley's work, including his lyrics, in his critical writings. Eliot famously critiqued Shelley, suggesting that his poetry, while passionate, lacked a certain maturity or intellectual rigor, which he characterized as adolescent.
This assessment contrasts with other critics who might have focused on different aspects of Shelley's poetry.
Therefore, T.S. Eliot is the critic associated with the specific phrase used in the question.